The theoretical acoustic sensing characteristics of a fiber laser are described based on Michelson interferometric demodulation technique.The fiber laser sensor is made by writing Bragg grating with π phase-shift into a section of Er-doped fiber.Acoustic pressure applied on the fiber laser induces laser wavelength shift.And a polarization-insensitive unbalanced Michelson fiber interferometer based on the 3×3 coupler is employed to transform the wavelength shift into phase shift of the interferometer.The signals are processed by Digital Signal Processing(DSP) to demodulate the phase shift.Based on the asymmetry characteristic of 3×3 coupler output signal,a novel algorithm for demodulating the phase shift is proposed.By employing two output signals whose amplitude can be adjusted in real time,it don't need uniform splitting ratio of coupler.The sensitivity experiment shows that the sensitivity is-166.5 dB(re 1 rad/μPa) when a 200 m path imbalanced readout interferometer is used,and the system output is linear to the pressure applied on the fiber laser sensor.