To improve the high frequency attenuation qualities of the acoustic low-pass filtering underwater fiber-optic acoustic sensors, a new anti-alaising filtering fiber-optic underwater acoustic sensor with a four-order acoustic low-pass filter is proposed, which is based on the two-order structure reported previously. The general low frequency lumped parameters model of the underwater acoustic sensor is constructed based on the theories of electro-acoustic analogy, and the acoustic performance is predicted by the circuit analysis methods. To test the model and the theories, a four-order acoustic low-pass filtering fiber-optic underwater acoustic sensor is fabricated and tested in a standing-wave tube filled with water. Apart from the hight frequency response, the experimental results are well in agreement with the numerical curve. The measured low frequency response is very flat, and the acoustic sensitivity is about -140 dB. The measured two resonant frequencies are in well agreement with the corresponding numerical values, respectively. It is expected that the acoustic low-pass filtering underwater fiber-optic acoustic sensor will become a new class of sensors used to eliminate the aliasing in the future sonar systems.