I am in Beijing stayed near Dongdaqiao Station, I intend to visit Great Wall.

I personally recommend you to visit Badaling Section. You can take bus no.919 from Dongzhimen Transport Hub to go to the Wall directly.
From Dongdaqiao, you may take subway line 6 for one stop to Chaoyangmen, where you can transfer to line 2. Get out from Exit B of Dongzhimen Station and you will find the transport hub to take that bus.

thanx Mr. Sam

What is the departure time of bus and i have metro smart card, can i use in this bus?
An other person told me go to Jishuitan Station and get 877 bus for Badaling. Now i am confused. Plz reply me

The smart card can still be used in bus.
I have seen a piece of new before stated that bus no.877 is not an official bus. This privately-run bus may provide poor service with high fare. It is not recommended to take it.