Can i take bus from Beijing Foreign Studies University to the Great Wall of Mutianyu ?

Of course you can, but the best choice is subway + bus. Walk from the campus to Weigongcun subway station to take line 4 to Xizhimen, where you may transfer to line 2 to Dongzhimen. There find bus no. 916 or Express 916 to Huairou North Avenue (Huairou Beidajie). Then, transfer to line h23, h24, h35, or h36 to Mutianyu Roundabout. Afterwards, walk about 500yd to the great wall.