First, you take the airport shuttle bus line 3 (Beijing International airport-Beijing Railway Station), arrive at Beijing Railway Station and take subway line 2 and get off at Jishuitan, walk about 10min and reach Deshengmen Gate, at Deshengmen Gate, you can take bus no.919 and arrive at Badaling Great Wall. The price of bus no. 919 costs CNY 12. Nice trip~