It is no use telling him not to worry.为什么tell不加不定式啊

It is no use doing sth 是一种固定结构,固定用法（干某事没有用处没有好处）,还有it's no good doing sth 等等doing 是动名词作真正的主语 而it is useful to do sth / it is important to do sth 等等,都是习惯用...