A modified contact model of joint interfaces based on fractal theory was proposed. Furthermore, numerical simulation was carried out to obtain the complicated nonlinear relations between normal contact stiffness and the normal contact load、the fractal dimension D and the fractal characteristic length scale parameter G. The results showed that the normal contact stiffness increased with the normal load, decreased with G but complicatedly with D. The results showed that the contact model could be used to calculate the normal contact stiffness and the normal load.