changsha airport to south railway station by bus

Well, there will be no bus to South Railway Station at midnight. But you can take a bus to the main railway station for CNY 15.5. The buses operate until the arrival of last flight, so you can catch it. Then, you can stay near the main railway station. On the next day, you can take metro line 2 from the main railway station to South Railway Station directly. ;)

Will i be able to catch the bus from airport to main railway station at midnight?

Yes, you can take airport shuttle bus to get to Main Railway Station at the midnight. It operates until the arrival of the last flight.