A novel signal detection method for interferometric fiber optic hydrophones is described. It compensates the phase of the interferometer by tuning the frequency of the laser. The principle of this method is introduced in details. The demodulation error is analyzed and simulated theoretically. A system of data acquisition and processing is programmed, and a fiber hydrophone is tested. The average acoustic pressure sensitivity is -162.2 dB (0 dB=1 rad/μPa) over the frequency range of 20 Hz to 1.3 kHz, and the fluctuation is smaller than ±0.8 dB, which accords well with the results obtained with the method of phase generated carrier demodulation. The experimental results prove this method is feasible. This method has the advantages of no electric elements in the sensing head, the simplicity of signal processing and the wide frequency range of detection. It can be applied to all kinds of signal detection of interferometric fiber-optic sensors.