Herein, an ultra-weak fiber Bragg grating hydrophone array system based on Fizeau interference is demonstrated. The ultra-weak fiber Bragg grating hydrophone array is constructed via on-line preparation of a fiber Bragg grating using a drawing tower. The amplitude, frequency, and phase of underwater acoustic signals in a wide frequency range of 2-2000 Hz are simultaneously measured using the signal demodulated by the arc-tangent algorithm. The phase acoustic-pressure sensitivity of the system hydroacoustic signals at 2 Hz reaches -135.81 dB(re rad/μPa). The sensitivity is 2755.49 μPa/ Hz, and the signal-to-noise ratio is 43.785 dB. The experimental results of sensor multiplexing prove that the system can demodulate underwater acoustic information at different locations. Furthermore, the phase acoustic-pressure sensitivity of the system increases with the increasing cavity length of the sensor. The system exhibits very good stability, indicating the possibility of extensive applications in high-sensitivity underwater acoustic sensing, very low frequency acoustic detection, deep-sea monitoring, and other fields.