diff --git a/alien.py b/alien.py
index 2c65aa7..6fff291 100644
--- a/alien.py
+++ b/alien.py
@@ -30,4 +30,6 @@ class Alien(Sprite):
         """如果外星人位于屏幕边缘，就返回True"""
         screen_rect = self.screen.get_rect()
         if self.rect.right >= screen_rect.right:
+            return True
+        elif self.rect.left <= 0:
             return True
\ No newline at end of file