The extrinsic Fabry Perot interference scheme was used to design an acoustic wave sensor based on chromium gold composite metal thin film. At the same time, the phase demodulation method based on fast Fourier transform was used to demodulate the white light interference between the optical fiber end face and the energy conversion film, which improved the phase sensitivity. The experimental results show that the sensitivity of the sensor remains in the range of -119.98~-120.99dB re 1rad/μPa in the range of sound wave frequency of 5~1000Hz, and the sensitivity change is less than 1.01dB, with a wide response flat range. It works stably in frequency 150Hz and in the range of sound pressure 0.18~5.30Pa, and there is a good linear relationship between sound pressure and phase.