Thanks to those who have contribute to this forum. I have book a minibus but cancel it because of adverse weather. In the end we decide to take two taxis(20RMB/taxi) from Sunworld Hotel(Dongcheng District) to Deshengmen Gate and then board bus 919(12RMB/pax). A woman bus ticket seller in blue uniform will collect money and issue bus ticket to you inside the bus. The bus 919 will stop next to cable car. You have to walk about 10 minutes to the Wall ticket office and Wall entrance. From our hotel to Badaling the whole journey took us one and a half hour. We climb all the way to the eighth tower and then took a cable car down. You just have to walk passed a row of souvenirs shops to reach the bus 919 station.