at在介词里边是是指在某一点（刚好就是那个点），on是指在某一点以上的地方 on the top of 在物体外部的上面at the top of 在物体内部的上面例如表示屋顶上的东西时用on房子里的日光灯就要用at
at the top of：
1.I waited at the top of the stairs. 我在楼梯顶端等着。
2.He stopped at the top of the stairs. 他在楼梯顶端停了下来。
3.They paused at the top of the stairs, doubtful as to which way to go next 他们在楼梯顶部停住了，不知道接下来该往哪边走。
4.Who's at the top of your speed-dial list? 你的一键拨号名单上第一个是谁？
5.Someone had played a trick on her, stretched a piece of string at the top of those steps.有人搞恶作剧整她，在最高的台阶上拉了一根绳子。
on the top of:
1.You will notice that your baby has two soft areas on the top of his head. 你会注意到你的宝宝头顶上有两处柔软的地方。
2.At last they saw him emerge on the top of an eminence within hail. 最后，他们看到他在一个山岭上出现了，置身于欢呼之中。
3.She is the first woman who steps on the top of Mount qomolangma. 她是第一个登上珠穆朗玛峰的女人。
4.Clouds are forming on the top of the hill. 云正在山顶上形成。
5.The climbers stood on the top of the mountain, gazing at the splendid view. 登山队员们站在山顶，凝视那壮丽的景色。