An unbalanced M-Z interference system based on the 3×3 coupler is established. Combining the 3×3 coupler demodulation with the phase carrier, the output phase of the M-Z interferometer is dynamically demodulated by introducing carrier to adjust the output phase. The carrier wave is a cosine signal with a frequency of 2 kHz and a voltage of 0, 2, 6, 10 and 16 V. The output power curve of the interferometer with time is measured by the Throlabs optical energy meter. For the unequal splitting ratio of 3×3 coupler with 1 ∶ 1 ∶ 1, the phase difference is not 120°, the output interference light is decomposed by empirical mode, and a band-pass filter is constructed for denoising. The processed signal is fitted with Lissajous curve by the least square method to obtain the specific parameters of ellipse. The arctangent function is used to solve the phase difference. Finally, the voltage phase shift coefficient and the voltage wavelength shift coefficient are 0. 757 7 (°/ V) and 0. 725 8×10 -3 ( nm/ V), respectively. The first-order linear curve is fitted in MATLAB, and the goodness of fit R 2 is more than 0. 98. Finally, the demodulation system is applied to the stress load test of fiber Bragg grating (FBG) strings and compared with the micron optics (MOI) demodulation. The results show that the accuracy of this system is high, and the FBG demodulation effect at 1 560 nm is better than MOI.