In order to improve measurement resolution of RLG(Ring laser gyroscope), the mantissa subdivision method was used and its influence on the accuracy of the strapdown inertial navigation system was analyzed. Firstly, the analysis of the traditional four sub-frequency subdivision method shows that its measurement resolution is a quarter period. Secondly, the principle, realization and characteristics of the mantissa subdivision method which is based on four sub-frequency subdivision method were contrastively analyzed. It drew a conclusion that the new method can obviously improve the measurement resolution of RLG. Based on the conclusion above, influences on the accuracy of the strapdown inertial navigation system of the two methods are analyzed by Simulink. The simulation indicates that the mantissa subdivision method can improve the navigation accuracy comparing with the traditional four sub-frequency subdivision method. Therefore, the mantissa subdivision method is meaningful to both theoretic study and engineering applications of high precision navigation system.