For a material, the design of a reasonable surface roughness structure is a main method to increase the contact angle between it and the liquid, increasing the hydrophobic of the material. In this paper, the two contact state relationships and contact angle on the solid surface, including Wenzel and Cassie states, are analyzed theoretically, and the effect of the fractal structure on the two contact models is compared. A good hydrophobic effect can be obtained by modifying the rough structure of Cassie contact state. The influence of nofractal structure and the existence of twolevelfractal structure on the contact angle of Cassie state are analyzed by using molecular dynamics model. The contact angles of the two contact states increase with the modification of fractal structure, especially the increase of Cassie state. The simulation results show that when the two dimensional fractal structure exists on the solid surface, the contact angle increases by 5°compared with the surface without fractal structure. The error between simulation value and theoretical value is about 1％. A good hydrophobic effect can be obtained by modifying the rough structure of Cassie contact state by using fractal structure in certain fractal series.