A novel mechanical anti-aliasing fiber-optic hydrophone with an acoustic low-pass filter of side cavities has been reported，to the best of our knowledge，for 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 analogy，and 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 cavities，and the hydrophone has good high frequency attenuation properties. A hydrophone with side cavities，designed and fabricated by us，is tested in a standing-wave tube filled with water. The measured frequency response curve，on the whole，has identical form as that predicted by theory, over the test frequency range of 50 to 7000 Hz. To be speaific，the low frequency responses are in good accordance，and the acoustic sensitivity is about -140 dB re 1 rad/μPa. The high frequency responses have obvious difference，which 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.