Abstract: According to the principle of the surface acoustic wave (SAW) effect, a technology has been developed to detect the low-frequency underwater acoustic signal by laser and a detection apparatus is developed. In dozens of Hertz low-frequencies, the surface wave produced by the underwater acoustic was detected. In the measurement process, the diffraction pattern was scanned by using MATLAB to analyze. Then the diffraction pattern pixel of stripe was obtained. According to the analytic formula of the wavelength and stripe space, the low-frequency liquid surface wavelength with millimeter order was obtained. By using computer programming, the regression curve was fitting according to the principle of least square method. Measurement results agree with the theoretical dispersion relationship. The method has the characteristics of real-time and non contact.