Can I get from Nansha in Guangzhou to Dongguan by long distance Bus?

As I know, the Baxianling Park is in SZ, so I think you are going to SZ instead of Dongguan.
You are suggested to take metro line 4 from Nansha Passenger Port to Chebei South to change to line 5 to Guangzhou Railway Station. Then, walk to the Bus Station near the station to take a bus to SZ. After arrival, take a taxi to your destination.