Hydroacoustic sensor of the passive optical fiber grating sensitivity enhanced through side pressure and end surface pulling is investigated. The filling and the elastic slice is used to enhance the sensitivity of the optical fiber grating sensor. The research results indicate that the resonance frequency using the filling method is too low (300 Hz), and high frequency sensitivity is too low (<-205 dB) too. The technology is complex. The Be-Cu (beryllium-copper) elastic slice is used to enhance sensitivity. In a frequency range of 100~1000 Hz the sensitivity reaches (-175±2) dB. The casing of the enhanced sensitivity sensor is made and applied to water sound detection. The results indicate that its sensitivity and frequency response can meet the application requirement.