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。