Hello, I want to travel from Shijiazhuang to Cangyanshan and I saw there is a direct bus from 8 AM to this place but how can I come back?

You can take the same bus to go back to the city proper. You can ask the staff to buy a round trip ticket, so that you can take the returning bus after visiting.
The returnig time is around 15:30.
In order to make sure, it is still suggested to consult the staff when you buy the tickets.

BTW, they start selling tickets at around 06:00, so you need to go there earlier.