i want to go Nepal from chengdu in metro passing over tibet...

I believe you are traveling to Lhasa by train, right? Well, it is safe enough. Don't worry! But there is only one train available for this route, taking almost 43 hours, so you are highly suggested to have a sleeper. At Lhasa Bus Station, you can take a bus to Zhangmu and then transfer to another bus to Nepal.

Using the way above, it takes much time and the route is a little complicated. If you wanna save the troubles, go by air.