Abstract: A method of detecting liquid surface wave based on laser diffraction was proposed. A He-Ne laser beam was obliquely incident upon liquid surface wave directly, the wave intensity was controlled by an electromagnetic type exciter and the frequency of the liquid wave was controlled by a signal generator, the laser diffraction patterns from liquid surface waves were collected by a charge-coupled device. The dependence of the diffracted field on the surface waves was derived by theory of wave optics. The experiment result indicates that the proposed method has the advantages of high sensitivity, high speed and non-destructive.