The four-mode laser gyro′s null shift is very complex under the high-low temperature process. It often vibrates when temperature changes linearly,which is a very difficult problem that cannot be explained and solved. Beam intensity changing along with temperature is simulated by changing the discharge current at room temperature,and the null shift also vibrates. So it can be concluded that temperature change induces beam intensity change,and then the null shift vibrates. If beam intensity of four-mode laser gyro can be kept constant in the high-low temperature,the null shift vibration will be effectually controlled. The conclusion is proved by the high-low temperature experiment with intensity stability.