A novel mechanical anti-aliasing fiber-optic hydrophone with an acoustic low-pass filter of side cavities has been reportedto the best of our knowledgefor the first time. The low frequency lumped parameters model and the effective acoustic circuit of the hydrophone is constructed based on the theories of electro-acoustic analogyand the acoustic performance is predicted by the circuit analysis methods. Theoretical results show that the hydrophone has three resonant frequencies and one inverse resonant frequency induced by the side cavitiesand the hydrophone has good high frequency attenuation properties. A hydrophone with side cavitiesdesigned and fabricated by usis tested in a standing-wave tube filled with water. The measured frequency response curveon the wholehas identical form as that predicted by theory, over the test frequency range of 50 to 7000 Hz. To be speaificthe low frequency responses are in good accordanceand the acoustic sensitivity is about -140 dB re 1 rad/μPa. The high frequency responses have obvious differencewhich is mainly induced by the operation limitation of the low frequency model. It is expected that this new type of fiber-optic hydrophone will work as an important class of sensors used to eliminate the aliasing in the future fiber-optic sonar systems.