A novel approach to PM fiber length error measuring of IFOG′s Sagnac interferometer was proposed according to the FOG manufacturing process. Basing on the state four square wave mechanism, the measurement of fiber length error was conversed into the measurement of peak pulses in the detector′s response signal. Meanwhile, fine measurement of the peak pulse was accomplished by modulation frequency multiplying technique. A fiber length error measuring system was developed on the basis of this approach, which achieved the measuring precision of 0.2 m(2 000 m fiber). The experimental results show that a higher precision and a faster measuring speed than original methods, which can increase the efficiency of FOG′s engineering assembly.