The position and intensity of polarization coupling points in polarization-maintaining fibers（PMFs） caused by stress and twist can be detected effectively by Polarization Coupling Measurement（PCM）,so the distributed PMFs sensing system of stress,strain,temperature and position can be realized.However,the Signal Noise Ratio（SNR） and the detection sensitivity of Polarization Coupling Measurement will decrease due to various noise sources.In order to improve SNR of the measurement system,external carrier modulation using LiNbO3 modulator is implemented.The modulated signal is received by the photodiode and digital demodulation algorithm is used for data processing.The experimental results show that the measurement sensitivity is obviously improved when scanning speed is 0.75 mm/s,modulation frequency is 9.1 kHz and samples frequency is 140 kHz.The gain of SNR is 8 dB and the demodulation algorithm spends only 1.12 s.