Dear Tan Mee Wan,
Greetings from Sophie, Top China Travel.
Thank you very much for the question.
If you take flight to Changsha, and hope to travel to Zhangjiajie by bus, you will need to take taxi or airport shuttle bus from Changsha airport to Changsha West Bus Station to take bus to Zhangjiajie. The bus departs every half an hour between 07:20 and 20:30. It is about 390km from Changsha to Zhangjiajie.
If you can speak Chinese, it is easy to find the West Bus Station to catch the bus for Zhangjiajie. But if you can not speak Chinese, I do suggest you fly to Zhangjiajie airport directly. It is easier and more convenient. There are flights to Zhangjiajie from Beijing, Shanghai, Xian, Guilin, ect.
Best regards
Sophie