Abstract: Preload relaxation is one of the main causes affecting the durability of bolted composite joints. This paper focuses on the interaction between material creep and contact creep of rough surface which induces the preload relaxation. The content includes:A creep constitutive model of composites based on the elastic-viscoplastic theory was established and extended to the elastic-viscoplastic contact problem with time-dependent combined with the fractal contact theory of rough surface. The comparison between numerical results and experimental results shows that the calculation error decreases from 2.87%-4.37% to 0.04%-0.5% when considering the rough surface contact effect, whose accuracy is significantly improved. The discussion results of the surface fractal parameters D and G show that the rougher the contact surface is, the easier the preload will be to relax, which will be of guiding significance to improve the surface contact properties by controlling surface topography parameters.