In this paper, the calculation formulas of the asperity’s deformation related with the surface contact pressure are deduced by using the simplified contact model. Firstly, we assume that the rough surface is composed of a set of cones as asperities, and the cones are arranged in different ways along two directions. Secondly, according to the mechanical analysis of a rigid conical punch on a half-space, the theoretical relationship between the average pressure of the micro contact area and the property parameters of a conical punch is obtained. Meanwhile, the calculation formula of the average pressure is given under the reasonable assumptions, which is related with the asperity’s deformation and the contact pressure. Finally, combining two theoretical relationships above, the quantitative analysis method for micro asperity’s deformation of two rough surfaces in contact is provided by using the average pressure as a connection bridge.