diff --git a/problems/signals.py b/problems/signals.py
index 50e8031..e2be8b9 100644
--- a/problems/signals.py
+++ b/problems/signals.py
@@ -45,16 +45,18 @@ def reset_user_quotas_on_change(sender, instance, created, **kwargs):
         return
     
     # 延遲導入避免循環依賴
+    from django.db import transaction
     from submissions.models import UserProblemQuota
     
     # 重置所有該題目的配額記錄
-    updated_count = UserProblemQuota.objects.filter(
-        problem_id=instance.id,
-        assignment_id__isnull=True  # 只重置全域配額，不影響作業配額
-    ).update(
-        total_quota=new_quota,
-        remaining_attempts=new_quota
-    )
+    with transaction.atomic():
+        updated_count = UserProblemQuota.objects.filter(
+            problem_id=instance.id,
+            assignment_id=None  # 只重置全域配額，不影響作業配額
+        ).update(
+            total_quota=new_quota,
+            remaining_attempts=new_quota
+        )
     
     logger.info(
         f"Problem {instance.id} total_quota changed from {old_quota} to {new_quota}. "
diff --git a/submissions/views.py b/submissions/views.py
index 32c3069..f36e864 100644
--- a/submissions/views.py
+++ b/submissions/views.py
@@ -886,15 +886,32 @@ def post(self, request, *args, **kwargs):
             if problem_quota >= 0:
                 # 題目有配額限制，需要檢查/建立該用戶的 UserProblemQuota 記錄
                 with transaction.atomic():
-                    quota, created = UserProblemQuota.objects.select_for_update().get_or_create(
-                        user=user,
-                        problem_id=problem_id,
-                        assignment_id=None,  # 全域配額（非作業）
-                        defaults={
-                            'total_quota': problem_quota,
-                            'remaining_attempts': problem_quota
-                        }
-                    )
+                    try:
+                        # 先嘗試在行鎖下取得現有配額紀錄
+                        quota = UserProblemQuota.objects.select_for_update().get(
+                            user=user,
+                            problem_id=problem_id,
+                            assignment_id=None,  # 全域配額（非作業）
+                        )
+                        created = False
+                    except UserProblemQuota.DoesNotExist:
+                        # 如不存在則嘗試建立，若發生競態導致 IntegrityError，再回頭取得
+                        try:
+                            quota = UserProblemQuota.objects.create(
+                                user=user,
+                                problem_id=problem_id,
+                                assignment_id=None,  # 全域配額（非作業）
+                                total_quota=problem_quota,
+                                remaining_attempts=problem_quota,
+                            )
+                            created = True
+                        except IntegrityError:
+                            quota = UserProblemQuota.objects.select_for_update().get(
+                                user=user,
+                                problem_id=problem_id,
+                                assignment_id=None,  # 全域配額（非作業）
+                            )
+                            created = False
                     
                     if quota.remaining_attempts == 0:
                         return api_response(
@@ -904,9 +921,11 @@ def post(self, request, *args, **kwargs):
                         )
                     
                     # 減少配額
-                    if quota.remaining_attempts > 0:
-                        quota.remaining_attempts -= 1
-                        quota.save()
+                    quota.remaining_attempts -= 1
+                    quota.save()
+                    
+                    # 儲存提交記錄，確保配額扣減與提交建立的原子性
+                    submission = serializer.save()
             else:
                 # 題目無配額限制 (total_quota == -1)
                 # 但仍檢查是否有管理員手動設定的 UserProblemQuota（針對特定用戶的限制）
@@ -915,7 +934,7 @@ def post(self, request, *args, **kwargs):
                         quota = UserProblemQuota.objects.select_for_update().get(
                             user=user,
                             problem_id=problem_id,
-                            assignment_id__isnull=True
+                            assignment_id=None  # 全域配額（非作業），與上方 get_or_create 邏輯一致
                         )
                         # 只有當 total_quota >= 0 時才檢查（-1 表示此用戶也無限制）
                         if quota.total_quota >= 0:
@@ -925,14 +944,15 @@ def post(self, request, *args, **kwargs):
                                     message="you have used all your quotas",
                                     status_code=status.HTTP_403_FORBIDDEN
                                 )
-                            if quota.remaining_attempts > 0:
-                                quota.remaining_attempts -= 1
-                                quota.save()
+                            quota.remaining_attempts -= 1
+                            quota.save()
+                        
+                        # 儲存提交記錄，確保配額扣減與提交建立的原子性
+                        submission = serializer.save()
                 except UserProblemQuota.DoesNotExist:
                     # 沒有任何配額限制，允許提交
-                    pass
+                    submission = serializer.save()
             
-            submission = serializer.save()
             
             # NOJ 格式響應
             return api_response(