The failure of plunger pair caused by insufficient back pressure was analyzed by using simulation of Pro/E and ADAMS. The simple 3d model of hydraulic motor was established with Pro/E, which was imported into ADAMS to set up dynamics model. The analysis of dynamic simulation was taken in the condition of the inlet pressure at 25 MPa, rated speed at 180 r/min, and outgoing pressure taking normal pressure at 06 MPa,045 MPa and the insufficient pressure at 02,01 and 005 MPa. The result of experiment indicates that the idler wheel breaks away from the guide rail in outgoing oilfield at insufficient pressure，and it strikes the guide rail in an instant of entering the incoming oilfield which causes very large impact force that easily damages the pistoncylinder pair.