You can take the airport bus line 2 and get off at Xidan. Then transfer subway line 4 and get off at beijing south railway station.

At the Airport, please take the Airport Express Train to its terminal at Dongzhimen firstly (25minutes)and then transfer to a taxi to the South Railway Station (20minutes).

For saving more, at Dongzhimen, please transfer to metro Line 2 in the station and then to Line 4 at Xuanwumen station. The metro Line 4 has a stop at South Railway Station, and the metro station is connected to the departure hall of the train station. The metro fare is only CNY and will take you about 30~40 minutes.