"I just ate lunch" 或者 "I have just had lunch" 都可以表示 "刚吃完午饭"。"I just ate lunch" 来自于英语中现在完成时的用法，它表示动作发生在过去，但对现在产生了影响。例如：

I just ate lunch, so I'm not hungry anymore. (我刚吃完午饭，所以我现在不饿了。)

I have just had lunch, so I'm feeling very full. (我刚吃完午饭，所以我感觉非常饱。)

另外，如果想要更简单地表示 "刚吃完午饭"，可以使用短语 "just finished lunch"，例如：

I just finished lunch, so I'm ready to get back to work. (我刚吃完午饭，所以我准备回去工作。)

I have just finished lunch, so I'm looking forward to a nice afternoon. (我刚吃完午饭，所以我期待着一个美好的下午。)