is there bus go to Xi'an from Wudangshan or shiyan? What's time departure?

From Wudangshan, there is no direct bus, but there are train services from Wudangshan railway station, which is not far from the mountain scenic area.
From Shiyan Bus Station, there are buses to Xian, and the bus schedule is at 08:00, 09:30, 11:00, 13:00, 15:00, 16:00 and 17:30.

where is Shiyen Bus Station that go to Xian

The bus station is about 700 yards walking distance from the square of train station, you will find it easily.