I would arrive Shenzhen airport midnight. What is the best way going to Dabu County?

As I know, no bus is available at that time, so you need to spend your night at the airport.
Next morning, you can take metro line 11 and switch to line 3 at Futian. Get off at Nanlian and leave from Exit B2, walk northeast for about 10 minutes to Longgang Bus Station. There are two directly buses to Dabu at 9:00 and 18:30. It costs CNY150 and takes about 6h.