The earliest hour to go to Beijing South Railway Station?

Hi, I'm not sure whether the shuttle bus departing at 07:00 from the airport is early enough for you. Anyway, the airport shuttle bus line 2 operating from 07:00 to 24:00 can take you from the airport to Xidan. It takes about an hour or a little more. After getting off, you can walk to Xidan MTR Station and take subway line 4 to Beijing South Railway Station in about 10 min.

Here is another way for you. Airport Express is suggested cause it starts its operation at 06:35 (T2)and 06:21 (T3). So you can take the airport express to Dongzhimen first. Then you can take subway line 2 at Dongzhimen MTR Station and get off at Xuanwumen. After that, you can transfer to subway line 4 to Beijing South Railway Station very fast.