knock at the door ，敲门; 叫门; [例句]There had been a knock at the door and when she opened it she locked herself out.外面有敲门声，她去开门，结果把自己反锁在了外面。knock on the door 敲门声; 敲门; [例句]To his relief a loud knock on the door spared him from giving an explanation.一声重重的敲门声让他松了一口气，他可以不用解释了。at强调一个点,on强调面 ，二者区别不大。at 常和knock 名词形式连用，on常和knock 动词形式连用。如果knock用作名词时，用at。没有太绝对的区别，只是一般这样使用。