hello I plan to Muntianyu on March 28.2015.and is there any direct bus from Dongzhimen to reach

Well, there is no direct public tranportation that can take you to the scenic area from Dongzhimen.
You can take public bus no. 916 (Fast) to Huairou North Avenue Station.
Then take bus no. H24 (inter-zone vehicle), and get off at Mutianyu Roundabout Station. After arriving, walk to the scenic area directly.