close the door behind him 的字面意思确实是“关上他身后的门”，无论一个人通过门是“进来”还是“出去”，一旦他“进来”或“出去”后，门就在他身后了。所以将其翻译成“关上身后的门”也不算错，不过更多是将其翻译成“随手把门关上”“随手关上身后的门”。

Harry stepped into the passage and closed the door behind him.

网友可以把behind him(在他后面的)理解介词短语作后置定语修饰前面名词the door.   即the door behind him在那身后的那扇门。

behind在…身后（随手关上门）【英文解释: When you shut a door or gate behind you, you shut it after you have gone through it.】

I walked out and closed the door behind me.我走了出来，随手带上了门。

He slammed the gate shut behind him.  他砰的一声带上了大门。