The geoacoustic is mainly infrasonic wave whose frequency is concentrated in 0~20 Hz, which has high requirements on the sensitivity, accuracy and frequency response characteristics of the sensor, reasonable signal demodulation method is the key of sensor research. A dual wavelength single channel quadrature demodulation method is proposed. According to the interference principle of light, the sensitive mechanism of the extrinsic geoacoustic sensor based on extrinsic Fabry-Perot interferometric(EFPI) is theoretically analyzed, it makes clear that the sensor needs to meet the requirements in the linear region when working, and the simulation analysis is carried out by using Matlab software. The simulation results show that the algorithm can avoid the influence of external environment on the output results, and can accurately demodulate the original signal, which proves the feasibility of the demodulation method.