Multiple joints such as fault F13 developed in K1 350～400 of a diversion tunnel of hydropower station in the southwest, which was deep-buried with high crustal stress and complex geological conditions. Using three-dimensional discrete element software 3DEC, the numerical simulation on the diversion tunnel is conducted combining with field monitoring data to analyze the stability of surrounding rock. The results show that the stability of surrounding rock is mainly controlled by the structural plane. There are many fall-blocks at the lower right spandrel with larger displacement. After being supported, the surrounding rock stability of the diversion tunnel is strengthened.