A new type of optical fiber underwater acoustic sensor (hydrophone) constructed with two fiber Bragg gratings (FBG) with signal self-demodulation function is presented. This fiber Bragg grating hydrophone uses a pair of matched fiber Bragg gratings with push-pull structure for both sensing and demodulating, which greatly reduces system complexity. The cylindrical structure of the hydrophone improves the sensitivity of the bare fiber Bragg grating acoustic sensors and also can compensate temperature. Simulation and preliminary experimental results indicate that the dynamic range of the hydrophone unit is beyond 100 dB and the measurement sensitivity can reach 0.36 nm/MPa for underwater acoustic pressure range 100～200 dB re 1 μPa.