Distributed optical fiber sensing system can be used for monitoring in long-distance and strong EMI environment. A method for external modulation with phase modulator is proposed to improve positioning precision of distributed optical fiber sensor. Construct an all-fiber vibration sensing system based on Michelson interferometer

and a phase modulator is attached to reflective end to modulate

so interfered signal reflected by reflective end in high frequency

and other signals remain in low frequency. Through high pass filtering and phase demodulation

signal which is proportional to external disturbance is acquired

and precision of positioning can be largely improved with this signal. This method is quite simple and easy to achieve

and through theoretical analysis and experimental results show that this method can effectively improve positioning precision.