diff --git a/alien.py b/alien.py
index 9dc38b3..541ef39 100644
--- a/alien.py
+++ b/alien.py
@@ -35,4 +35,5 @@ class Alien(Sprite):
             return True
 
     def update(self):
-        """向左或右移动外星人"""
\ No newline at end of file
+        """向左或右移动外星人"""
+        self.x += (self.ai_settings.alien_speed_factor *
\ No newline at end of file