In order to improve the sensitivity and accuracy of fiber Bragg grating (FBG) monitoring micro-displacement, a substrate-based FBG displacement sensor is proposed and implemented. The sensing is realized based on beat-frequency signals of different modes in the resonator. The Sagnac ring cavity effectively improves the utilization of the pump light source, and has smaller phase noise and higher signal-to-noise ratio than the straight cavity. The substrate-based strain sensing structure can effectively protect the optical fiber and accurately measure the frequency variation caused by the displacement. In the experiment, the drift of the sensing signal is observed every 5 mm. The results of repeated experiments are recorded. The results show that the frequency shift varies linearly with the displacement, the degree of linear fit is up to 0.9995, and the sensitivity is -45.4 kHz/mm. According to the highest precision of the spectrometer, we obtain that the measurement accuracy of the sensor is 0.88×10 -3 mm, which is basically consistent with the theoretical derivation.