Abstract: The self-cleaning of the solid surface is an attracting field, and so far there is not enough knowledge about its microscopic mechanism. In this paper, with ZnO as the substrate, the influence of the nanostructure size on its self-cleaning capability is discussed and the optimal nanostructure model is obtained by using the molecular dynamics simulation to simulate the self-cleaning effect on the nanostructure. It is found that the optimal sizes of the pillar-type solid surfaces for self-cleaning are 7.05 Å, 7.38 Å, 2.16Å as the pillar height, the pillar side length and the pillar side margin, respectively, while the contact angle is 140.8°. The result also shows that the simulated contact angle agrees better with the predicted contact angle in the Cassie model than that in the Wenzel model.