diff --git a/packages/application/auth/wechat_sync_use_case.py b/packages/application/auth/wechat_sync_use_case.py
index 76d04e326..eae3b17e1 100644
--- a/packages/application/auth/wechat_sync_use_case.py
+++ b/packages/application/auth/wechat_sync_use_case.py
@@ -34,7 +34,7 @@ class WechatSyncRequest:
     ):
         self.openid = openid.strip()
         self.unionid = unionid.strip() if unionid else ""
-        self.nickname = nickname or "微信用户"
+        self.nickname = nickname or "小虾同学"  # 微信新规拿不到真实昵称，新用户默认昵称
         self.avatar_url = avatar_url or ""
         self.source = source
 
@@ -227,7 +227,7 @@ class WechatSyncUseCase:
             id=user_id,
             email=email,
             username=username,
-            display_name=request.nickname or "微信用户",
+            display_name=request.nickname or "小虾同学",
             password_hash=password_hash,
             email_verified=True,  # 微信登录视为已验证
             wechat_openid=request.openid,
diff --git a/tests/unit/test_wechat_sync_use_case.py b/tests/unit/test_wechat_sync_use_case.py
index b19d212c3..eb12e4598 100755
--- a/tests/unit/test_wechat_sync_use_case.py
+++ b/tests/unit/test_wechat_sync_use_case.py
@@ -60,7 +60,8 @@ class TestWechatSyncRequest:
         assert WechatSyncRequest(openid="o1", unionid="  unionid_456  ").unionid == "unionid_456"
 
     def test_default_nickname(self):
-        assert WechatSyncRequest(openid="o1").nickname == "微信用户"
+        # 微信新规拿不到真实昵称，新用户统一默认"小虾同学"
+        assert WechatSyncRequest(openid="o1").nickname == "小虾同学"
 
     def test_default_source(self):
         assert WechatSyncRequest(openid="o1").source == "miniapp"
@@ -319,3 +320,78 @@ class TestWechatSyncSession:
         assert kw["user_id"] == "user_001"
         assert "wechat_miniapp" in kw["device_info"]
         assert kw["expires_in_seconds"] == 30 * 24 * 3600
+
+
+class TestWechatSyncNicknameRules:
+    """昵称规则：默认昵称、登录不覆盖存量昵称、中文昵称全链路无损。"""
+
+    def test_existing_user_display_name_never_overwritten_on_login(self, mock_user_repo, mock_session_store):
+        """守护规则：已存在用户登录时，即使请求带了别的 nickname，
+        其 display_name 也绝不能被覆盖（只有建号时写入）。"""
+        user = User(
+            id="user_custom",
+            email="c@wechat.local",
+            username="wx_custom",
+            display_name="我的专属昵称",
+            password_hash="h",
+            email_verified=True,
+            wechat_openid="openid_123",
+            wechat_unionid="unionid_456",
+        )
+        mock_user_repo.find_by_wechat_openid.return_value = user
+        mock_user_repo.find_by_wechat_unionid.return_value = user
+
+        resp, err = make_use_case(mock_user_repo, mock_session_store).execute(
+            WechatSyncRequest(
+                openid="openid_123",
+                unionid="unionid_456",
+                nickname="小虾同学",  # 请求里带的任何昵称都不准覆盖
+            )
+        )
+        assert err is None
+        assert resp.is_new_user is False
+        # 数据层用户昵称不变
+        assert user.display_name == "我的专属昵称"
+        # 返回给调用方的昵称也是存量昵称
+        assert resp.nickname == "我的专属昵称"
+        # save 的都是同一个用户对象，且 display_name 未被改写
+        for call in mock_user_repo.save.call_args_list:
+            assert call.args[0].display_name == "我的专属昵称"
+
+    def test_new_user_gets_default_nickname_when_nickname_empty(self, mock_user_repo, mock_session_store):
+        """新用户且请求未带昵称（微信新规场景）→ 默认昵称小虾同学。"""
+        saved = {}
+        mock_user_repo.save.side_effect = lambda u: saved.update({u.id: u})
+        resp, err = make_use_case(mock_user_repo, mock_session_store).execute(
+            WechatSyncRequest(openid="new_oid_default", unionid="new_uid_default", nickname="")
+        )
+        assert err is None
+        assert resp.is_new_user is True
+        u = saved[resp.user_id]
+        assert u.display_name == "小虾同学"
+        assert resp.nickname == "小虾同学"
+
+    def test_chinese_nickname_full_chain_no_mojibake(self, mock_user_repo, mock_session_store):
+        """中文昵称全链路无损：建号写入 → 仓储对象 → 响应返回，
+        不允许出现 latin-1/utf-8 双重编码乱码（如 \u00e8\u0080\u0081 一类）。"""
+        chinese_name = "老字号🏮肩腰腿🏮御众堂永康店"
+        saved = {}
+        mock_user_repo.save.side_effect = lambda u: saved.update({u.id: u})
+
+        resp, err = make_use_case(mock_user_repo, mock_session_store).execute(
+            WechatSyncRequest(
+                openid="new_oid_cn",
+                unionid="new_uid_cn",
+                nickname=chinese_name,
+            )
+        )
+        assert err is None
+        u = saved[resp.user_id]
+        # 写入仓储对象的昵称必须与原文完全一致
+        assert u.display_name == chinese_name
+        assert resp.nickname == chinese_name
+        assert resp.to_dict()["user"]["display_name"] == chinese_name
+        assert resp.to_dict()["user_info"]["display_name"] == chinese_name
+        # 乱码特征：双重编码后必然出现Ã/\u00c2 拼音字母或控制字符
+        assert "\u00c3" not in u.display_name and "\u00c2" not in u.display_name
+        assert "\u00e8" not in u.display_name