Abstract: By taking Youshashan fault in Yingdong area of Qaidam Basin as an example, based on the developed formation model, this paper analyses the shale smear of the fault and makes fitting calculation of the fault zone displacement pressure in order to find a feasible method for fault sealing ability evaluation in condition of large fault throw. It is suggested that reasonable development of the fault zone model and proper division of the layers are the premise of the fault sealing ability evaluation, proper utilization of the evaluation means is the key of it. The study shows that the depth of about 2 000 m is a node for sealing of Youshashan fault; the fault is partially open above the depth and completely closed below it. It is concluded that deep footwall of Youshashan fault could have good prospects for petroleum exploration