In order to solve the problem of frequency identification of the low frequency water surface acoustic wave (WSAW), a laser interferometric detection method is proposed based on an improved phase-generated carrier wave (PGC) demodulation technique. A simple laser interferometer system is used to detect the low frequency WSAW caused by underwater acoustic sources. Frequency mixing of four carrier signals is conducted and two orthogonal interference signals are selected by power comparison. Further phase demodulation and spectrum analysis are carried out, and the frequency measurement for WSAW is achieved. Simulations and experiments for low frequency WSAW are conducted to confirm that the improved PGC demodulation method can effectively avoid orthogonal signal blanking when the initial phase of carrier signal is uncontrollable. The accurate frequency identification for WSAW can be achieved, and the frequency detection limit is as low as 30 Hz. Good detection effect for WSAM with low frequency can be achieved by the proposed method under conditions of frequency-varying WSAM and low-frequency WSAM with large-scale disturbance wave interference. The results show that the improved PGC demodulation method can accurately determine the frequency of WSAW and be able to resist disturbance.