Abstract: In view of the serious problem of cavitation of external gear pump in open hydraulic system, the process of cavitation and the measures to solve the cavitation phenomenon are studied. By analyzing the flow of oil in the pump, the causes of cavitation of the pump are obtained. According to the reasons of cavitation, measures to reduce the cavitation of the gear pump by rectangular relief groove, involute relief groove and special relief groove are proposed, and the performance is compared by numerical simulation. Finally, the oil trapping phenomenon and volumetric efficiency of the pump under different relief groove are analyzed. The results show that the high-speed swirling and suction in the oil suction chamber is closely related to the occurrence of cavitation in the gear pump; The cavitation intensity and range are different when the gears are in different meshing states; There are between the trapped oil phenomenon and the volumetric efficiency of the pump under the action of three different unloading tanks. The conclusion investigated in this study will provide much scientific information for the solving cavitation phenomenon of gear pump and choosing relief groove shape.