A laser interference detection method based on phase demodulation had been developed to measure the amplitude of low frequency water surface acoustic waves and identify the changed sound source. Water surface acoustic wave amplitude measurement was achieved by introducing a high frequency carrier into the reference optical path, using the arctangent demodulation algorithm of phase-generated carrier technology, in combination with the determined of initial phase and the modulation depth of the carrier. A set of amplitude measurement system was constructed under laboratory conditions, and the experimental results show that this method can accurately identify the amplitude, and the measurement repeatability is better than 1 nm. The detection result of the amplitude-modulated sound source signal indicates that the method can accurately track the change of the water surface acoustic wave amplitude, which proves the correctness of the method for identifying the changed sound source.