Abstract: In this study, the complex rough surface is simplified as the superposition of a series of trigonometric curves based on the least square method. In order to calculate the interface contact stiffness, the optimal substrate modeling thickness is determined by the elastic-plastic finite element method. The relationship between normal contact pressure and normal contact deformation and normal contact stiffness is quantitatively investigated. The mechanism of fractal dimension and characteristic scale parameters of rough surface on normal contact stiffness is revealed. The results show that there is an optimal thickness of the substrate, which can effectively improve the calculation efficiency of the contact stiffness of the rough surface. The normal contact stiffness increases nonlinearly with the increase of normal contact deformation and normal contact pressure. The surface fractal dimension and characteristic scale parameters have significant effects on the normal contact stiffness, and the normal contact stiffness increases with the increase of the fractal dimension. However, the normal contact stiffness decreases with the increase of characteristic scale parameters.