It is of great sidnificance for the strapdown inertial navigation equipment to keep the zero bias constant in the long working process of laser gyro，this paper introduces the main factors that affect the change of zero bias of laser gyro and analyzes the mechanism that causes the change of zero bias in the working process of laser gyro. Through the simulation calculation and experimental verification，the results show that the internal aeration pressure of laser gyro has little effect on the zero bias.The proper partial pressure ration is helpful to reduce the influence of material air leakage on zero bias drift，the optimal inflation parameters are given through experiments，which is of great significance to the engineering of laser gyro.