The changes of SLD's spectral modulation in fiber-optic gyroscope was studied with experiment,the reason for the increasing of spectral modulation depth through the whole optical path was explained in theory,and the emulation result was highly consistent with the experimental result. On the basis of that,the influence of spectral modulation on the coherence function was analyzed,and the error level of fiberoptic gyroscope which arise from secondary coherence peak was evaluated.