The fundamental step to acquire the rotation rate of fiber optic gyro (FOG) is to detect the differential optic phase. The characters of FOG’s output signal which are with large noise, nonlinear, periodic and their consequences to the detection of the effective signal are analyzed briefly. Here a tracing of differential optic phase is presented and sin modulation is adopted in the method, triangular operator is applied and finally the rotation signal is gotten after multipling on FOG output and the tracing signal, and adding filters to where they are needed. With this method the dynamic range and the linear of scale factor of FOG are improved. It also has the advantages of simple, low cost and easy to accomplished. The phase tracing simulation and the test circuit are made and the results are given.