A new variable displacement oil pump has been modelled and simulated by using AMESim. Through the comparison and analysis of the simulation curve and the test data of this pump, it is found that the volume efficiency of it is unsatisfactory when the rotational speed is too high. The mechanical structure model of the pump was established by using ADAMS to confirm that the cause of the problem is redundant force on the variable device. On this basis, the redundant force acting on the variable device is theoretically deduced to prepare for the improvement of the pump.