Beijing capital airport to Tien An Men

At that time, there is no shuttle bus or the local subway system in service. So at least you need to stay in the airport for one hour for the first airport express.
You can take the airport express to Dongzhimen Subway Station and change to bus line 2 to Jianguomen Station and then change to line 1 to East Tiananmen Station. Get out of the station, you can walk to the Tiananmen Square.

If you are hurry to reach that square, you are suggested to take a taxi to the square directly.