Surrounding rock mass stability is one of the key technical problems in the design of the diversion tunnels in Jinping II Hydropower Plant Project. The major difficulties lie in three facts: (1) high stress induced by the great depth; (2) the brittle failure characters of marble; (3) the interaction of these four tunnels. A systemic procedure is introduced in this paper. Firstly, the integration of the code FLAC3D and the multivariate linear regression method is adopted to back analyze the in situ stress field. Then the mechanical parameters of the surrounding rock mass are back analyzed based on the PSO (Particle Swarm Optimization) algorithm and the code FLAC3D. The stress release method is adopted in the numerical analysis of the excavating and supporting process of tunnels by the code FLAC3D. Finally, the multi indexes integration method is presented to analyze and evaluate the stability of the tunnels and to validate the rationality of the design scheme.