diff --git a/yudao-module-mall/yudao-module-trade/src/main/java/cn/iocoder/yudao/module/trade/service/cart/CartServiceImpl.java b/yudao-module-mall/yudao-module-trade/src/main/java/cn/iocoder/yudao/module/trade/service/cart/CartServiceImpl.java
index 9b57b32..37e309e 100644
--- a/yudao-module-mall/yudao-module-trade/src/main/java/cn/iocoder/yudao/module/trade/service/cart/CartServiceImpl.java
+++ b/yudao-module-mall/yudao-module-trade/src/main/java/cn/iocoder/yudao/module/trade/service/cart/CartServiceImpl.java
@@ -46,14 +46,14 @@ public class CartServiceImpl implements CartService {
     public Long addCart(Long userId, AppCartAddReqVO addReqVO) {
         // 查询 TradeCartDO
         CartDO cart = cartMapper.selectByUserIdAndSkuId(userId, addReqVO.getSkuId());
-        // 校验 SKU
-        Integer count = addReqVO.getCount();
+        // 校验 SKU：已存在时，按合并后的数量校验库存 https://github.com/YunaiV/ruoyi-vue-pro/issues/1267
+        Integer count = cart != null ? cart.getCount() + addReqVO.getCount() : addReqVO.getCount();
         ProductSkuRespDTO sku = checkProductSku(addReqVO.getSkuId(), count);
 
         // 情况一：存在，则进行数量更新
         if (cart != null) {
             cartMapper.updateById(new CartDO().setId(cart.getId()).setSelected(true)
-                    .setCount(cart.getCount() + count));
+                    .setCount(count));
             return cart.getId();
         // 情况二：不存在，则进行插入
         } else {
diff --git a/yudao-module-mall/yudao-module-trade/src/test/java/cn/iocoder/yudao/module/trade/service/cart/CartServiceImplTest.java b/yudao-module-mall/yudao-module-trade/src/test/java/cn/iocoder/yudao/module/trade/service/cart/CartServiceImplTest.java
new file mode 100644
index 0000000..b512151
--- /dev/null
+++ b/yudao-module-mall/yudao-module-trade/src/test/java/cn/iocoder/yudao/module/trade/service/cart/CartServiceImplTest.java
@@ -0,0 +1,190 @@
+package cn.iocoder.yudao.module.trade.service.cart;
+
+import cn.iocoder.yudao.framework.test.core.ut.BaseMockitoUnitTest;
+import cn.iocoder.yudao.module.product.api.sku.ProductSkuApi;
+import cn.iocoder.yudao.module.product.api.sku.dto.ProductSkuRespDTO;
+import cn.iocoder.yudao.module.product.api.spu.ProductSpuApi;
+import cn.iocoder.yudao.module.trade.controller.app.cart.vo.AppCartAddReqVO;
+import cn.iocoder.yudao.module.trade.controller.app.cart.vo.AppCartUpdateCountReqVO;
+import cn.iocoder.yudao.module.trade.dal.dataobject.cart.CartDO;
+import cn.iocoder.yudao.module.trade.dal.mysql.cart.CartMapper;
+import org.junit.jupiter.api.Test;
+import org.mockito.ArgumentCaptor;
+import org.mockito.InjectMocks;
+import org.mockito.Mock;
+
+import static cn.iocoder.yudao.framework.test.core.util.AssertUtils.assertServiceException;
+import static cn.iocoder.yudao.module.product.enums.ErrorCodeConstants.SKU_NOT_EXISTS;
+import static cn.iocoder.yudao.module.product.enums.ErrorCodeConstants.SKU_STOCK_NOT_ENOUGH;
+import static org.junit.jupiter.api.Assertions.*;
+import static org.mockito.ArgumentMatchers.any;
+import static org.mockito.Mockito.*;
+
+/**
+ * {@link CartServiceImpl} 的单元测试
+ *
+ * @author 芋道源码
+ */
+public class CartServiceImplTest extends BaseMockitoUnitTest {
+
+    @InjectMocks
+    private CartServiceImpl cartService;
+
+    @Mock
+    private CartMapper cartMapper;
+    @Mock
+    private ProductSpuApi productSpuApi;
+    @Mock
+    private ProductSkuApi productSkuApi;
+
+    @Test
+    public void testAddCart_insert() {
+        // 准备参数
+        Long userId = 1L;
+        AppCartAddReqVO reqVO = new AppCartAddReqVO().setSkuId(10L).setCount(3);
+        // mock 方法
+        when(productSkuApi.getSku(eq(10L))).thenReturn(new ProductSkuRespDTO().setId(10L).setSpuId(20L).setStock(10));
+        when(cartMapper.insert(any(CartDO.class))).thenAnswer(invocation -> {
+            invocation.<CartDO>getArgument(0).setId(100L);
+            return 1;
+        });
+
+        // 调用
+        Long cartId = cartService.addCart(userId, reqVO);
+        // 断言
+        assertEquals(100L, cartId);
+        ArgumentCaptor<CartDO> cartCaptor = ArgumentCaptor.forClass(CartDO.class);
+        verify(cartMapper).insert(cartCaptor.capture());
+        CartDO cart = cartCaptor.getValue();
+        assertEquals(userId, cart.getUserId());
+        assertEquals(20L, cart.getSpuId());
+        assertEquals(10L, cart.getSkuId());
+        assertEquals(3, cart.getCount());
+        assertTrue(cart.getSelected());
+        verify(cartMapper, never()).updateById(any(CartDO.class));
+    }
+
+    @Test
+    public void testAddCart_mergeBelowStock() {
+        // 准备参数
+        Long userId = 1L;
+        AppCartAddReqVO reqVO = new AppCartAddReqVO().setSkuId(10L).setCount(2);
+        // mock 数据
+        CartDO dbCart = new CartDO().setId(100L).setUserId(userId).setSpuId(20L).setSkuId(10L)
+                .setCount(3).setSelected(false);
+        when(cartMapper.selectByUserIdAndSkuId(eq(userId), eq(10L))).thenReturn(dbCart);
+        // mock 方法
+        when(productSkuApi.getSku(eq(10L))).thenReturn(new ProductSkuRespDTO().setId(10L).setSpuId(20L).setStock(10));
+
+        // 调用
+        Long cartId = cartService.addCart(userId, reqVO);
+        // 断言
+        assertEquals(100L, cartId);
+        ArgumentCaptor<CartDO> cartCaptor = ArgumentCaptor.forClass(CartDO.class);
+        verify(cartMapper).updateById(cartCaptor.capture());
+        CartDO cart = cartCaptor.getValue();
+        assertEquals(100L, cart.getId());
+        assertEquals(5, cart.getCount());
+        assertTrue(cart.getSelected());
+        verify(cartMapper, never()).insert(any(CartDO.class));
+    }
+
+    @Test
+    public void testAddCart_mergeEqualsStock() {
+        // 准备参数
+        Long userId = 1L;
+        AppCartAddReqVO reqVO = new AppCartAddReqVO().setSkuId(10L).setCount(4);
+        // mock 数据
+        CartDO dbCart = new CartDO().setId(100L).setUserId(userId).setSpuId(20L).setSkuId(10L)
+                .setCount(6).setSelected(true);
+        when(cartMapper.selectByUserIdAndSkuId(eq(userId), eq(10L))).thenReturn(dbCart);
+        // mock 方法
+        when(productSkuApi.getSku(eq(10L))).thenReturn(new ProductSkuRespDTO().setId(10L).setSpuId(20L).setStock(10));
+
+        // 调用
+        Long cartId = cartService.addCart(userId, reqVO);
+        // 断言
+        assertEquals(100L, cartId);
+        ArgumentCaptor<CartDO> cartCaptor = ArgumentCaptor.forClass(CartDO.class);
+        verify(cartMapper).updateById(cartCaptor.capture());
+        assertEquals(10, cartCaptor.getValue().getCount());
+    }
+
+    @Test
+    public void testAddCart_mergeOverStock() {
+        // 准备参数：本次数量、已有数量都未超过库存，但合并后超过
+        Long userId = 1L;
+        AppCartAddReqVO reqVO = new AppCartAddReqVO().setSkuId(10L).setCount(3);
+        // mock 数据
+        CartDO dbCart = new CartDO().setId(100L).setUserId(userId).setSpuId(20L).setSkuId(10L)
+                .setCount(8).setSelected(false);
+        when(cartMapper.selectByUserIdAndSkuId(eq(userId), eq(10L))).thenReturn(dbCart);
+        // mock 方法
+        when(productSkuApi.getSku(eq(10L))).thenReturn(new ProductSkuRespDTO().setId(10L).setSpuId(20L).setStock(10));
+
+        // 调用，并断言异常
+        assertServiceException(() -> cartService.addCart(userId, reqVO), SKU_STOCK_NOT_ENOUGH);
+        // 断言：未写入，且原购物项未被修改
+        verify(cartMapper, never()).updateById(any(CartDO.class));
+        verify(cartMapper, never()).insert(any(CartDO.class));
+        assertEquals(8, dbCart.getCount());
+        assertFalse(dbCart.getSelected());
+    }
+
+    @Test
+    public void testAddCart_skuNotExists() {
+        // 准备参数
+        Long userId = 1L;
+        AppCartAddReqVO reqVO = new AppCartAddReqVO().setSkuId(10L).setCount(1);
+        // mock 数据
+        CartDO dbCart = new CartDO().setId(100L).setUserId(userId).setSpuId(20L).setSkuId(10L)
+                .setCount(1).setSelected(true);
+        when(cartMapper.selectByUserIdAndSkuId(eq(userId), eq(10L))).thenReturn(dbCart);
+        // mock 方法：SKU 已被删除
+        when(productSkuApi.getSku(eq(10L))).thenReturn(null);
+
+        // 调用，并断言异常
+        assertServiceException(() -> cartService.addCart(userId, reqVO), SKU_NOT_EXISTS);
+        // 断言：未写入
+        verify(cartMapper, never()).updateById(any(CartDO.class));
+        verify(cartMapper, never()).insert(any(CartDO.class));
+    }
+
+    @Test
+    public void testUpdateCartCount_equalsStock() {
+        // 准备参数
+        Long userId = 1L;
+        AppCartUpdateCountReqVO reqVO = new AppCartUpdateCountReqVO().setId(100L).setCount(10);
+        // mock 数据
+        when(cartMapper.selectById(eq(100L), eq(userId))).thenReturn(new CartDO().setId(100L).setUserId(userId)
+                .setSpuId(20L).setSkuId(10L).setCount(3).setSelected(true));
+        // mock 方法
+        when(productSkuApi.getSku(eq(10L))).thenReturn(new ProductSkuRespDTO().setId(10L).setSpuId(20L).setStock(10));
+
+        // 调用
+        cartService.updateCartCount(userId, reqVO);
+        // 断言
+        ArgumentCaptor<CartDO> cartCaptor = ArgumentCaptor.forClass(CartDO.class);
+        verify(cartMapper).updateById(cartCaptor.capture());
+        assertEquals(100L, cartCaptor.getValue().getId());
+        assertEquals(10, cartCaptor.getValue().getCount());
+    }
+
+    @Test
+    public void testUpdateCartCount_overStock() {
+        // 准备参数
+        Long userId = 1L;
+        AppCartUpdateCountReqVO reqVO = new AppCartUpdateCountReqVO().setId(100L).setCount(11);
+        // mock 数据
+        when(cartMapper.selectById(eq(100L), eq(userId))).thenReturn(new CartDO().setId(100L).setUserId(userId)
+                .setSpuId(20L).setSkuId(10L).setCount(3).setSelected(true));
+        // mock 方法
+        when(productSkuApi.getSku(eq(10L))).thenReturn(new ProductSkuRespDTO().setId(10L).setSpuId(20L).setStock(10));
+
+        // 调用，并断言异常
+        assertServiceException(() -> cartService.updateCartCount(userId, reqVO), SKU_STOCK_NOT_ENOUGH);
+        // 断言：未写入
+        verify(cartMapper, never()).updateById(any(CartDO.class));
+    }
+
+}