How can i go reach changsha south station to airport?

There is no direct mode. Upon arrival, you can take the subway line 2 to Main Railway Station. Then take the Exit 1 and walk about 300 yards to Civil Aviation Hotel (Minhang Hotel) to take the shuttle bus to the airport.

Or you can take a taxi the whole way, and it may cost you CNY60.

Thanks a lot wandy for making things clear!