A fractal unloading model of joint interfaces was established based on fractal contact theory and unloading model of asperity. The unloading of asperities both elastic and elastic-plastic deformation was taken into account in the model. By numerical simulation, the nonlinear relationship among the real contact areas, contact force, contact pressure and interference was obtained, as was as the difference between loading and unloading. The result showed that the unloading process of joint interfaces was elastic, and depended on the final condition of loading. During the unloading process, the real contact area and contact force depended on the interference of joint interface. When the loading process started, both the real contact area and contact force sharply decreased and smaller than the values during loading. As the interference increased, the real contact area became larger than the value during loading soon, while the contact force was smaller than the value during loading in a wide range of interference. The unloading contact pressure was much less than the loading contact pressure within the entire interference interval. 