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The motion compensation based on ring laser gyroscope (RLG) position and orientation system (POS) is a key technology to improve the imaging quality and efficiency of airborne Earth observation system. However, RLG POS faces great problems in the vibration environment, where the mechanical dither of RLG causes the adverse disturbance to inertial measurement unit (IMU) which should be eliminated, and the external vibration must be accurately measured with high bandwidth and low noise. To solve the problem, a kinetics model of RLG IMU is established based on a vibration response mechanism in this paper. An optimized design method of mechanically dithered RLG IMU with a vibration-damping system is proposed that can reduce the measurement error. In addition, the size-effect error and optimization method of RLG IMU are analyzed. Based on finite-element analysis software, a high-precision mechanically dithered RLG POS is designed and developed to be used in various imaging payloads. The experimental results show that the inertial navigation errors of RLG POS are 0.196 (σ), 0.659 (σ), and 0.707 kn (σ) for static, vehicle, and airborne experiments, respectively. It contributes to realize high-quality image of airborne interferential synthetic aperture radar and 1 : 500 scale map of camera without ground control marks. The precision of developed RLG POS is close to that of the most advanced production POS/AV610.