diff --git a/alien.py b/alien.py
index 541ef39..bd12f5e 100644
--- a/alien.py
+++ b/alien.py
@@ -36,4 +36,5 @@ class Alien(Sprite):
 
     def update(self):
         """向左或右移动外星人"""
-        self.x += (self.ai_settings.alien_speed_factor *
\ No newline at end of file
+        self.x += (self.ai_settings.alien_speed_factor *
+        self.ai_settings.fleet_direction)
\ No newline at end of file