Based on the acousto-optic diffraction theory, the light diffraction characteristics of low-frequency liquid surface waves was investigated using the method of optical. An expression was given about the non-corresponding orders, the analytical expression was simulated by means of the numerical method, which explains the mechanism of the non-correspondence effect. In experiment, the steady and visible diffraction pattern was observed, and the fringe spacing has obvious asymmetry. With the increasing of incident angle, the distinct non-correspondence of the positive and negative diffraction orders is found. The number of the positive diffraction pattern is more than the negative one. The results show that: the non-correspondence effect is related to the incident angle. With the increasing of incident angle, the non-correspondence effect is more obvious. The negative diffraction order has maximum. For the orders exceeding this value, the negative diffraction pattern disappeared. It shows that the theoretical analysis agrees with the experimental result well.