how can go to great wall at BaDaLing via subway, in Beijing city, china?

Actually, no direct subway can take you to Badaling Great Wall. Here are the transport means:
Many buses are on schedule at local North Railway Station to Badaling Great Wall. Near the railway station - Xizhimen MTR Station is convenient and subway line no. 2, 4 and 13 can take you to Xizhimen MTR Station.
Besides, you can also go to Deshengmen to take bus no. 919 to Badaling directly.