Abstract: The research results of a passive fiber grating hydrophone based on polymer side-hole package structure are reported. The pressure sensitivity of fiber grating with side-hole package was 17120 times of that without package; this practice establishes the physical foundation for the underwater acoustic signal being detected by fiber grating. Using the narrowband width and tunable wavelength property of laser, an intensity modulation scheme with work-point controlling was drawn, which has eliminated the influence of temperature change on the measurement results, and the dynamic signal detection with high accuracy is realized. The experimental result shows that, the acoustic sensitivity of fiber grating hydrophone reaches -153.3 dB (0 dB=-1 pm/μPa) at the frequency of 500 Hz and has a good isotropy directivity property. The minimal detected sonar pressure of the hydrophone is 61 dB (0 dB=1μPa/√Hz).