Modeling of random drift and compensating in navigation system is the key of improving the performance of marine FOG. In the paper, Allan variance method that can be applied to the random drift research of any precision instrument is modified with the application of window function to resolve the non-unique characterization of random drift caused by frequency alias. The measured data of FOG are researched and various coefficients of random drift are also obtained. The experiments have proved that this method is effective to evaluate FOG random drift.