Effects of liquid viscosity coefficient on acoustic properties of acoustic low-pass filtering fiber-optic hydrophone are studied theoretically and experimentally. For better descriptions, the key parameter of machanical acoustic resistance which is used to describe the mechanical loss of the system, is introduced into the previous lumped parameters model. Then the improved expression of the transfer function is obtained by the circuit analysis method. Simulation results show that the liquid viscosity coefficient mainly affects the response near the resonant frequency. The resonant frequency is approximately identical and the transfer function amplitude at the resonant frequency decreases sharply, with the increasing of the liquid viscosity coefficient. Acoustic sensitivity frequency responses of different temperature are measured in the standing-wave tube filled with castor oil. The experimental results are in well agreement with the simulation results.