Hi, I plan to go from Xian to Zhangjiajie ,which train that i should use ?

Well, there is one direct train to Zhangjiajie from Main Railway Station: Train T284. It departs at 01:17, and arrives at 19:10 (about 18 hours).

Yes, you can make a transfer at Zhengzhou. There are many bullet trains to Zhengzhou from North Railway Station, such as G2002, G652, G820, etc. The trip takes about 2.5 hours.

There are two trains to Zhangjiajie from Zhengzhou. Train K967 departs at 18:22, and arrives at 10:48 (about 16.5 hours). Train K267 departs from 23:10, and arrives at 11:48 (about 12.5 hours).

You can choose the way you like.