I intend to go to great wall from agricultrue exhibition centre subway, Beijing

Which section of Great Wall are you going to visit?
If it is Badaling, walk from where you are to Liangmaqiao Bus Stop to take bus no. 909 to Deshengmen where find bus no. 919 or 880, either of which can take you to Badaling Great Wall directly.

By suway, you need to take line 10 at Agricultural Exhibition Center Station to Beitucheng where transfer to line 8 to Guloudajie Station. It is connected to Jishuitan by line 2. Getting off at Jishuitan to walk to Deshengmen to find bus no. 919 or 880 to the destination.

If you are going to Mutianyu Wall, take bus no. 635 to Dongzhimen, where there are bus no. 867 taking you there directly.