A simple measurement system is developed to study the optical effect of the low-frequency liquid surface wave which excited by underwater acoustic source. The high stability and clear diffraction pattern was observed experimentally.The relationship between diffraction patterns divergence angle and the distance of the acoustic signal was derived. Furthermore, with the increase of distance, the diffraction patterns divergence angle will decrease. The damping characters of the liquid surface wave was theoretically obtained when underwater acoustic wave spread to the liquid surface.The analytical expression between diffraction patterns divergence angle and the liquid surface wave amplitude was theoretically derived. It was found that the surface wave amplitude is exponent attenuation with the change of the horizontal distance.The attenuation coefficients is dependent on the frequency of liquid surface acoustic wave, and the greater frequency, the smaller attenuation coefficient.