Aiming at the phase reversal phenomenon in the demodulation process of ultra weak fiber grating hydrophone array, a phase compensation method based on the one- hot finite state machine is proposed to modify the flip signal in realtime in field programmable gate array. The phase of the demodulated signal and its compensation conditions are set in a one-hot coded finite state machine, and the phase compensation of the demodulated signal of the optical fiber interference system is realized through the instantaneous state transition of the state machine at a high clock rate. The power consumption, occupied resources and timing results in the functional simulation and experimental test of various phase compensation methods are compared and analyzed. The results show that the phase compensation method based on the one-hot finite state machine can not only solve the phase reversal problem correctly and ensure the integrity of the signal, but also increase the dynamic range of signal demodulation, and reduce the logic delay of the system by 6%. It has advantages in the high throughput and high clock frequency applications of underwater acoustic fiber sensing demodulation system.