Variable oil pump changes the flow by changing the mating length of gear of its variable device, and then the fuel consumption of the engine was reduced. During the design process of the gear pump, the rules of changes of flow, the displacement, pressure oscillation during operation and drive torque of the gear were considered comprehensively. Nephogram of changes along with time of physical flow, pressure oscillation and cavitation was visually shown by the CFD analysis software Pumplinx. Both of the maximum and minimum displacement of the pump was selected for the CFD analysis, and the flow characteristics of flow, torque and cavitation of the pump were gotten. By comparing the analysis results and the experimental data, the analysis results match well the range required by SOR, thus the performance of the variable gear pump is studied even better.