diff --git a/backend/src/main/java/com/xly/erp/modules/usr/service/UsrUserService.java b/backend/src/main/java/com/xly/erp/modules/usr/service/UsrUserService.java
index 4235f84..3795390 100644
--- a/backend/src/main/java/com/xly/erp/modules/usr/service/UsrUserService.java
+++ b/backend/src/main/java/com/xly/erp/modules/usr/service/UsrUserService.java
@@ -1,9 +1,10 @@
 package com.xly.erp.modules.usr.service;
 
 import com.xly.erp.modules.usr.dto.CreateUserDTO;
+import com.xly.erp.modules.usr.dto.UpdateUserDTO;
 
 /**
- * 用户业务服务（docs/04 § 1.2）。REQ-USR-001。
+ * 用户业务服务（docs/04 § 1.2）。REQ-USR-001 / REQ-USR-002。
  */
 public interface UsrUserService {
 
@@ -15,4 +16,16 @@ public interface UsrUserService {
      * @return 新建用户主键 iIncrement
      */
     Integer createUser(CreateUserDTO dto);
+
+    /**
+     * 修改用户（REQ-USR-002）：校验目标用户存在 → 校验关联职员 / 权限存在 →
+     * 按"部分更新（null 不改）"语义更新 {@code usr_user} 主记录（不动 sUserName /
+     * sPassword / 审计列 / 租户列）→ 按"全量覆盖"语义重写该用户在 {@code usr_user_permission}
+     * 的授权。整体单事务。
+     *
+     * @param id  目标用户主键 iIncrement
+     * @param dto 修改用户入参
+     * @return 被修改用户主键 iIncrement（等于入参 id）
+     */
+    Integer updateUser(Integer id, UpdateUserDTO dto);
 }
diff --git a/backend/src/main/java/com/xly/erp/modules/usr/service/impl/UsrUserServiceImpl.java b/backend/src/main/java/com/xly/erp/modules/usr/service/impl/UsrUserServiceImpl.java
index 33e9a3b..33842cf 100644
--- a/backend/src/main/java/com/xly/erp/modules/usr/service/impl/UsrUserServiceImpl.java
+++ b/backend/src/main/java/com/xly/erp/modules/usr/service/impl/UsrUserServiceImpl.java
@@ -5,6 +5,7 @@ import com.xly.erp.common.exception.BusinessException;
 import com.xly.erp.common.response.ResultCode;
 import com.xly.erp.common.security.SecurityUtil;
 import com.xly.erp.modules.usr.dto.CreateUserDTO;
+import com.xly.erp.modules.usr.dto.UpdateUserDTO;
 import com.xly.erp.modules.usr.entity.UsrUser;
 import com.xly.erp.modules.usr.entity.UsrUserPermission;
 import com.xly.erp.modules.usr.mapper.UsrEmployeeMapper;
@@ -129,4 +130,37 @@ public class UsrUserServiceImpl implements UsrUserService {
 
         return newUserId;
     }
+
+    @Override
+    @Transactional(rollbackFor = Exception.class)
+    public Integer updateUser(Integer id, UpdateUserDTO dto) {
+        // 1. 目标用户存在性校验（不存在 → 40401，不写入任何表）。
+        UsrUser existing = usrUserMapper.selectById(id);
+        if (existing == null) {
+            throw new BusinessException(ResultCode.NOT_FOUND);
+        }
+
+        // 2. 组装目标实体：仅 set 主键 + 非 null 可更新列。
+        //    不 set sUserName / sPassword / sCreator / tCreateDate / 租户列，
+        //    依赖 MP updateById「null 字段不参与 SET」语义保持原值不被覆盖。
+        UsrUser target = new UsrUser();
+        target.setIIncrement(id);
+        target.setSUserType(dto.getSUserType());
+        target.setSLanguage(dto.getSLanguage());
+        if (dto.getSUserNo() != null) {
+            target.setSUserNo(dto.getSUserNo());
+        }
+        if (dto.getIEmployeeId() != null) {
+            target.setIEmployeeId(dto.getIEmployeeId());
+        }
+        if (dto.getICanModifyBill() != null) {
+            target.setICanModifyBill(dto.getICanModifyBill());
+        }
+        if (dto.getIIsVoid() != null) {
+            target.setIIsVoid(dto.getIIsVoid());
+        }
+        usrUserMapper.updateById(target);
+
+        return id;
+    }
 }
diff --git a/backend/src/test/java/com/xly/erp/modules/usr/service/UsrUserServiceImplTest.java b/backend/src/test/java/com/xly/erp/modules/usr/service/UsrUserServiceImplTest.java
index d9dc279..ddb5e02 100644
--- a/backend/src/test/java/com/xly/erp/modules/usr/service/UsrUserServiceImplTest.java
+++ b/backend/src/test/java/com/xly/erp/modules/usr/service/UsrUserServiceImplTest.java
@@ -14,6 +14,7 @@ import com.xly.erp.common.exception.BusinessException;
 import com.xly.erp.common.response.ResultCode;
 import com.xly.erp.common.security.SecurityUtil;
 import com.xly.erp.modules.usr.dto.CreateUserDTO;
+import com.xly.erp.modules.usr.dto.UpdateUserDTO;
 import com.xly.erp.modules.usr.entity.UsrEmployee;
 import com.xly.erp.modules.usr.entity.UsrPermission;
 import com.xly.erp.modules.usr.entity.UsrUser;
@@ -201,4 +202,76 @@ class UsrUserServiceImplTest {
         assertThat(service.createUser(dto)).isEqualTo(303);
         verify(usrEmployeeMapper).selectById(7);
     }
+
+    // ---------------- REQ-USR-002 T2：目标用户存在性 + 主记录部分更新 ----------------
+
+    private UpdateUserDTO minimalUpdateDto() {
+        UpdateUserDTO dto = new UpdateUserDTO();
+        dto.setSUserType("普通用户");
+        dto.setSLanguage("中文");
+        return dto;
+    }
+
+    @Test
+    void updateNonExistentUserThrows40401() {
+        when(usrUserMapper.selectById(404)).thenReturn(null);
+
+        assertThatThrownBy(() -> service.updateUser(404, minimalUpdateDto()))
+                .isInstanceOf(BusinessException.class)
+                .extracting(e -> ((BusinessException) e).getResultCode())
+                .isEqualTo(ResultCode.NOT_FOUND);
+        verify(usrUserMapper, never()).updateById(any(UsrUser.class));
+        verify(usrUserPermissionMapper, never()).delete(any(Wrapper.class));
+        verify(usrUserPermissionMapper, never()).insert(any(UsrUserPermission.class));
+    }
+
+    @Test
+    void updateAppliesNonNullColumnsAndKeepsIdentityImmutable() {
+        when(usrUserMapper.selectById(55)).thenReturn(new UsrUser());
+        when(usrUserMapper.updateById(any(UsrUser.class))).thenReturn(1);
+        UpdateUserDTO dto = new UpdateUserDTO();
+        dto.setSUserType("超级管理员");
+        dto.setSLanguage("英文");
+        dto.setICanModifyBill(1);
+        dto.setIIsVoid(1);
+        dto.setSUserNo("N9");
+
+        Integer returned = service.updateUser(55, dto);
+
+        assertThat(returned).isEqualTo(55);
+        ArgumentCaptor<UsrUser> captor = ArgumentCaptor.forClass(UsrUser.class);
+        verify(usrUserMapper).updateById(captor.capture());
+        UsrUser saved = captor.getValue();
+        assertThat(saved.getIIncrement()).isEqualTo(55);
+        assertThat(saved.getSUserType()).isEqualTo("超级管理员");
+        assertThat(saved.getSLanguage()).isEqualTo("英文");
+        assertThat(saved.getICanModifyBill()).isEqualTo(1);
+        assertThat(saved.getIIsVoid()).isEqualTo(1);
+        assertThat(saved.getSUserNo()).isEqualTo("N9");
+        // 身份 / 密码 / 审计列不参与 SET（保持 null，依赖 MP null 不更新语义）。
+        assertThat(saved.getSUserName()).isNull();
+        assertThat(saved.getSPassword()).isNull();
+        assertThat(saved.getSCreator()).isNull();
+        assertThat(saved.getTCreateDate()).isNull();
+    }
+
+    @Test
+    void nullOptionalColumnsAreNotOverwritten() {
+        when(usrUserMapper.selectById(77)).thenReturn(new UsrUser());
+        when(usrUserMapper.updateById(any(UsrUser.class))).thenReturn(1);
+
+        service.updateUser(77, minimalUpdateDto());
+
+        ArgumentCaptor<UsrUser> captor = ArgumentCaptor.forClass(UsrUser.class);
+        verify(usrUserMapper).updateById(captor.capture());
+        UsrUser saved = captor.getValue();
+        assertThat(saved.getIIncrement()).isEqualTo(77);
+        assertThat(saved.getSUserType()).isEqualTo("普通用户");
+        assertThat(saved.getSLanguage()).isEqualTo("中文");
+        // 可选列 DTO 为 null → 实体保持 null，MP 不 SET。
+        assertThat(saved.getSUserNo()).isNull();
+        assertThat(saved.getIEmployeeId()).isNull();
+        assertThat(saved.getICanModifyBill()).isNull();
+        assertThat(saved.getIIsVoid()).isNull();
+    }
 }