A measurement system is developed to detect the low-frequency underwater acoustic signal. The clear and high stable diffraction pattern is observed experimentally when the laser beam illuminates on the liquid surface which is modulated by the underwater acoustic signal, the diffraction fringes is asymmetric distribution. The asymmetry of the position and intensity of the diffraction spot were explained with the laser oblique incidence. The experiment is consistent with the theory. The results show that it is asymmetric distribution of the diffraction light field which is modulated by low-frequency underwater acoustic signal.