A Michelson interferometer based fiber optic sensor was constructed with the application of a 3×3 coupler, faraday rotator, and other optical elements. Phase generated carrier method was applied to modulate the laser. The constructed fiber disturbance sensor was used to a long-distance field safety monitoring. According to the analysis of the interferometer system output, a signal component with the same frequency of the carrier was found in the output signal. Consequently, the carrier signal could be extracted synchronously with a narrow-band band pass filter with center frequency being set to the carrier frequency. When the extracted carrier signal was used to passive homodyne demodulation procedure of phase generated carrier method, the external disturbance signal could be acquired. The method of synchronous carrier extraction was introduced, the feasibility of this method was obtained through theoretical derivation, and the correctness of this theory was authenticated through software simulation and experiment. The low frequency influence to the extracted carrier was also discussed, and the solution was given for the limit of the influence cased by the low frequency signal. This method can be applied especially in the condition when modulation is set far from the output signal to make it hard to acquire the carrier signal.