diff --git a/backend/src/backend/platform.py b/backend/src/backend/platform.py
index 9c0e3bb..871e05d 100644
--- a/backend/src/backend/platform.py
+++ b/backend/src/backend/platform.py
@@ -854,18 +854,34 @@ async def add_member(
             status.HTTP_422_UNPROCESSABLE_ENTITY,
             "用户的平台角色行不存在或已被删除",
         )
-    duplicate = await session.scalar(
-        select(WorkspaceMembers.user_id).where(
+    # ``WorkspaceMembers`` 的主键是 ``(workspace_id, user_id)`` 复合 PK,
+    # 而 ``remove_member`` / ``delete_platform_employee`` 都是软删除 (保留行,
+    # 仅置 ``is_deleted=1``). 因此这里必须按主键查整行,而不是只看活跃行:
+    # 否则软删行会被 active-duplicate 检查漏过,然后 INSERT 直接撞 PK.
+    existing = await session.scalar(
+        select(WorkspaceMembers).where(
             WorkspaceMembers.workspace_id == workspace_id,
             WorkspaceMembers.user_id == payload.user_id,
-            WorkspaceMembers.is_deleted == 0,
         )
     )
-    if duplicate is not None:
-        raise HTTPException(
-            status.HTTP_409_CONFLICT,
-            "用户已是该 workspace 成员;workspace 角色继承自平台角色,"
-            "要变更请 PATCH /api/v1/platform/employees/{user_id} 修改 role_code",
+    if existing is not None:
+        if existing.is_deleted == 0:
+            raise HTTPException(
+                status.HTTP_409_CONFLICT,
+                "用户已是该 workspace 成员;workspace 角色继承自平台角色,"
+                "要变更请 PATCH /api/v1/platform/employees/{user_id} 修改 role_code",
+            )
+        # 复活软删除行. 保留 ``joined_at`` 作为历史记录;``role_id`` 重新继承
+        # 当前用户的平台角色 (用户在中间可能改过 platform_role);清掉
+        # ``deleted_at`` 标记本轮已不在软删状态.
+        existing.is_deleted = 0
+        existing.deleted_at = None
+        existing.role_id = role.role_id
+        existing.member_status = "active"
+        await session.flush()
+        await session.refresh(existing)
+        return _envelope(
+            context.request_id, member_payload(user, role, existing),
         )
     membership = WorkspaceMembers(
         workspace_id=workspace_id,