Hello,Mr.Kahavah
From Tsinghua University to Badaling Great Wall, you can first take bus no. 699 from the west gate of Tsinghua University to Shangdi Huandao East (上地环岛东) and then bus no. 448 outer ring from Shangdi Huandao East to Xisanqi Bridge North (西三旗桥北) and at last take bus no. 919 Express (919路大站快车) from Xisanqi Bridge North to Badaling.