have one's heart in one's mouth 心都提到嗓子眼了；提心吊胆；害怕；非常吃惊；吓得要命

【例句】
My heart was in my mouth when I was taking part in an exam. 　　
当我考试的时候我的心都提到嗓子眼了。

The little boy stood outside the door of the headmaster's room and had his heart in his mouth, afraid to go in.
小男孩担心吊胆地站在校长室门口，不敢进去。