Referring to the angular measurement of stabilized platform system with Ring Laser Gyro(RLG),high accuracy,high stability and the appropriate dynamic range are important design requirements.RENISHAW grating angular encoder was the better choices,and the cyclone FPGA chip was selected as signal-processing circuit.Angular encoder signal was synchronized,filtered digitally,doubled frequency and discriminated phase before it was counted.Interface circuit was designed to implement communication between FPGA and the Master-Computer.RLG was applied to calibrate the grating angular measurement.The calibration result shows that by using this set of relative angular measurement devices angular resolution of 3″,measurement accuracy of ±8″ and dynamic rate range of 0 °/s～360 °/s are obtained steadily and accurately.