What time airport express do i need to take for 8am flight at PEK?

The earliest schedule of the train from Dongzhimen is at 06:00. And the frequency is about 10 mins. It takes around half an hour on the way. But the time is a little bit tight even if you take the first train in the early morning.
You are adviced to arrive at the airport 2 hours in advance. So just take a taxi from the starting place to the destination directly.