drop/lower your guard
to stop being careful to avoid danger or difficulty
放松警惕
Once he knew I wasn't a journalist, he dropped his guard and even let me take a photograph of him.
当他知道我不是记者的时候,他立刻放松了警惕,甚至让我为他照相。
It's normal enough to drop your guard when you have reached a great objective.
当你完成一个重要任务后,总是会丧失警惕性,这很正常。