I need to go from aeroport to westin guangzhou at 21:30

Well, you can take airport shuttle bus line 6A. But the last schedule of this bus is at 22:00. So I don't think that you can catch it.
So you are suggested to take subway line 3 at the airport, and get off at West Linhe Station, Exit A. Then walk north around 220 yards along West Linhe Road. After that, walk east along Linle Road around 185 yards to your hotel.