How do I get from Changping to the Center of Beijing ?

What do you mean by the center of BJ? If you don’t mind, I’ll take Tian’anmen Square as the city center.
As I know, there is a metro station, Nanshao Station, 2km away from the west end of Changjin Road. You may take a taxi there at a cost of CNY13/person. Then you can take metro Changping Line to Xi’erqi Station and transfer metro line 13 to Xizhimen Station. Afterwards, you can take metro line 2 to Qianmen Station, Exit A. Upon arrival, you can find the square directly.