Signal aliasing is one of puzzling questions of a practical fiber-optic hydrophone system based on phase generated carrier (PGC) modulation-demodulation. The physical mechanism of signal aliasing is analyzed theoretically and simulated by computer. The results show that increasing the modulation frequency and lowering the sensitivity of the fiber-optic hydrophone cannot effectively resolve the signal aliasing induced by high-frequency interfering signal. A novel acoustic low-pass filtering scheme is proposed based on the principle of acoustic low-pass filters. A simple fiber-optic hydrophone with an acoustic low-pass filter is designed and fabricated. The acoustic pressure phase sensitivity is measured in a standard standing-wave duct. The experimental results show that the fiber-optic hydrophone has fine characteristics of acoustic low-pass filtering, and can restrain the high-frequency interfering signal strongly. The low-frequency acoustic sensitivity is about -140 dB (0 dB=1 rad/μPa), the attenuation of high frequency is bigger than 20dB. The scheme is very simple and economical, can be used to eliminate the signal aliasing in a practical phase generated carrier modulation-demodulation system effectively.