In order to avoid cavitation phenomenon effectively in oil suction chamber of gear pump,taking the two meshing radius of the driving gear and the driven gear as the boundary,a closed space of oil suction chamber, which included the meshing point and meshing radius,was constructed,the corresponding volume change rate was derived according to the method of swept area.According to the volume change rate of the oil suction chamber was equal to the exchange flow outside the chamber, the formula for dynamic pressure change and minimum value of medium in the oil suction chamber was derived based on full unloading of trappedoil, and the discriminant was established to avoid cavitation.Finally, instance operation and result analysis were done. The results show that dynamic change of the medium pressure in the chamber is caused by the change of the meshing position, the minimum value of pressure is obtained at the position of node and meshing coincidence;the inlet radius is the main factor to avoid cavitation phenomenon, and the correct choice can avoid cavitation effectively.Many factors are considered in the calculation of the minimum pressure, which provides a theoretical basis for the further study of the cavitation performance of the gear pump.