Understanding the contact characteristics of rough surfaces is essential to explain engineering phenomenon in interface. In order to improve accuracy of contact model, a novel simplified fully plastic contact model of sphere asperity was proposed considering material properties based on fractal theory. First, based on Von Mises yield criteria, maximum contact pressure factor was derived. Second, relationships taking into consideration strain hardening effects were proposed to describe contact area based on the fully plastic contact area index and contact pressure. Then, the critical interference at inception of fully plastic deformation was derived. Lastly, validations were conducted for different materials. The results show that the present work is remarkably consistent with experimental results and has higher accuracy than other models.