This work investigates the acoustic response of a fiber-optic hydrophone, which consists of a Fary-Perot cavity covered with a large-area and 100 nm-thick graphite diaphragm.The hydrophone demonstrates an acoustic sensitivity of~18 nm/Pa at the frequency of 3.5 kHz and a minimum detectable pressure of~73 μPa/Hz1/2 at 10 kHz.The easy fabrication, low-cost and high sensitivity of the hydrophone could make it of practical importance for underwater acoustic detection.