How to get from Beijing South Railway Station to Capital Airport?

You are suggested to take subway line 4 to Xuanwumen Station. Then transfer to subway line 2 to Dongzhimen Station. The duration is 30 mins. It costs CNY 5 per person. After arriving, take the airport express to the PEK directly. It takes around half an hour. And the ticket fare is CNY 25 per person.
Taking the airport shuttle bus is not recommended. The duration would be longer if there is traffic jam.
In addition, you can purchase the tickets when you arriving there. Don't worry.