Auto-compensating the drift in optical gyros by IMU rotating mechanism is an important way to enhance the navigation precision. The theory of auto-compensating can be clarified in the navigation error propagating functions. Because the attributes of optical gyro determines that rotation of gyro will inevitably bring extra error, the rotaion of the whole IMU is the only mechanism, An eight-180-degree rotating mechanism of IMU was proposed and illustrated in detail. Analyses and simulation show that errors from long term drift can be highly restrained, thus navigation accuracy of both position and attitude can be greatly enhanced.