There is no direct shuttle bus from Beijing airport to Beijing South Railway Station. Well, two ways for your reference:
1: take the airport shuttle bus Line 1 to arrive at Fangzhuang. It is 16RMB and takes about one hours. From Fangzhuang, it is suggested that you take a taxi to the railway station. The fare is around 15RMB.

2. take the airport shuttle bus Line 3 to arrive at Beijing Railway Station firstly. It is also 16RMB and takes about one hour. Then, catch the city-bus 692 at the east square of Beijing Railway Station, who could take you to Beijing South Railway Station directly. Please note, when you arrive, you have to take the special line of Beijing South Railway Station, which leads you to the south square of this station to catch the bullet train.

Actually, you could catch the coaches from Beijing Capital Airport driving to Tianjin directly~