Abstract: Introduced the principle of fiber-optic hydrophone and how it detected of sound pressure. We fabricated some units of fiber-optic hydrophone, whose size wasφ22 mm×100 mm. Test showed its phase sensitivity was-160 dB(ref 1 rad/Hz^1/2) and fluctuation was less than 3 dB and minimum measurable sound pressure was 43.5 dB (at 1 kHz), as much as sea-state zero(SSO). The correlation coefficient of signals, which were sampled by a fiber-optic hydrophone and a standard hydrophone, were over 0.94. A 23-element fiber optic hydrophone array was deployed at Bohai via 1 km fiber optic underwater cable on August, 2002. The sound source was air gun. The correlation coefficients of signals sampled by adjacent elements were over 0.98 and the array had a good beam directionality. Progress on fiber-optic hydrophone implied a great future for application.