processmaker's api ( how to get dynaform uid by process uid and task uid?)

I can get pro_uid and tsk_uid, but how to get its dynaform uid?

http://wiki.processmaker.com/3.0/REST_API_Designer#Get_Activity, get_activity's response have nothing to do with dynaform.

You can use this url to get the uids of steps of a task. A step is usually a dynaform, so that is where you would get the uid of the dynaform for a specific task that you are looking for. http://wiki.processmaker.com/3.0/REST_API_Designer#Get_Steps_for_Activity:.3C.2Fcode.3EGET.2Fproject.2F.7Bprj_uid.7D.2Factivity.2F.7Bact_uid.7D.2Fsteps

I would also encourage you to check out this url: http://wiki.processmaker.com/index.php/ProcessMaker_API

It has full details of all ProcessMaker rest api related stuff.