Based on the fractal geometry theory,considering the hardness of the asperity changes with the amount of deformation in elastoplastic deformation stage due to strain hardening,the fractal models of the singleload stiffness in the first and second elasticplastic deformation stages of the joint interface are established.The relation between the normal contact stiffness and contact area of a single asperity in the elasticplastic deformation stage is derived when hardness variation is taken into account.Then,the dimensionality of the relationship between the normal contact stiffness,contact area and contact load of the joint interface in the elasticplastic deformation stage is obtained.Through simulation analysis,the influence of relevant parameters on the normal contact stiffness of the joint interface was obtained.Simulation results show that when hardness variation is considered,there is a relationship between the dimensionless normal contact stiffness and the actual contact load and the actual contact area.The normal contact stiffness is increased with the increase of fractal dimension D,when the fractal dimension is fixed,the normal contact stiffness of the joint interface is increased with the increase of the profile scale parameter G.