1.当一个句子已经有一个动词了 2.一些固定的词和词组后面比如 feel like doing risk doing it is no use doing 列举一些常见的后面加doing的:keep enjoy excuse practice mind avoid appreciate risk consider can't help give up imagine finish think of suggest hear of be fond of insist on aim at 其实就是大多数介词后面~~ (我可是一个一个打上去的。。。) 3.表示主动 4.表示进行