In order to realize the Fabry-Perot (F-P) acoustic vibration sensing demodulation working under wide range of optical fiber F-P initial cavity length and cavity length shift, a kind of quadrature phase extracting method based on dual tunable lasers is proposed. The range of initial cavity length which keeps quadrature phase is calculated theoretically. The influence of initial cavity length drift on signal demodulation is studied by theoretical simulation and experiment. Initial cavity length drift of 0~2.2 μm is achieved by changing temperature. In this case, interference intensity demodulation method and quadrature phase extracting method are compared and analyzed: relative change of demodulation signal amplitude by quadrature phase extracting method is less than 5.3% ; the stability of demodulation signal amplitude by quadrature phase extracting method is 12.4 times as much as that by interference intensity demodulation method. Initial cavity length of 55~130 μm is realized by wavelength tuning compensation and relative change of demodulation signal amplitude is less than 4.9%.