Light diffraction from liquid surface acoustic waves at low-frequency was experimentally realized. In the experiment steady and visible diffraction patterns were obtained,and the obvious asymmetry of patterns was observed for the first time. Theoretically the approximate conditions of the surface wave diffraction were analyzed,and the corresponding analytic expression of angle width of diffraction patterns was derived,which explained the asymmetry distribution well. The symmetry distribution of diffraction pattern is a universal rule. And whether it can be observed obviously depends on the ratio of the wavelength of surface wave and light. The bigger the ratio is,the more obvious the asymmetry distribution is . Under the condition that the surface wavelength is close to the light wavelength,the diffraction patterns can be considered as a kind of symmetry distribution approximately.