Abstract: Three different manners of environmental factors' effects on the light intensity and bias of four-frequency laser gyro were summarized. Since both light variation and null drift were the laser gyro's response to environmental factors' effects, light intensity can reflect the environmental factors' effects on laser gyro's bias well. As a result, a novel approach of light intensity compensation of four-frequency laser gyro's bias based on BP neural network was presented. A BP neural network of laser gyro's bias was set up. Then the light intensity compensation experiments for three different mechanisms of generating null drift, including startup null drift, mode hopping, and high-low temperature process, were carried out. It can be concluded that the approach can distinguish above three different mechanisms and compensate for their effects. The approach is meaningful to enhancing the laser gyro's performance under the complex environment.