Abstract: When the shear strength (c0, tan?0) of tunnel surrounding rock is reduced non-proportionally to the limit, there are countless possible critical instability failure states. Therefore, the strength reduction shortest path theory is introduced, and the upper bound method with rigid translatory moving elements (UB-RTME) is applied. Using mesh updating, inheritance strategy, and nonlinear programming computation, the critical unit weight γcr is solved incrementally to approach the actual unit weight γ0. This enables the non-proportional strength reduction analysis of circular tunnel surrounding rock. The results indicate: (1) The proportional reduction results from the UB-RTME match well with those calculated by the Optum G2 software, confirming the reasonableness of the surrounding rock slip line net failure mode; (2) Different strength reduction ratios ξ and reduction paths L can invert the ultimate state strength parameters (cs, tan?s), the comprehensive safety factor Fs, and their corresponding surrounding rock slip line net. As the reduction ratio ξ increases, the safety factor Fs and reduction path L first decrease and then increase,and the corresponding slip line net′s main failure surface shrinks inward, intensifying the dislocation effect between the rigid blocks representing failure characteristics; (3) The slip line net corresponding to different ultimate state points（cs/c0, tan?s/tan?0) has the same internal dissipated energy and resistance to failure effects. The ultimate line can distinguish the failure domain from the safety domain, but cannot identify the most unfavorable instability failure state of the surrounding rock according to the upper bound theorem; (4) The strength reduction shortest path theory shows that there is always a shortest reduction path Lmin in the limit state lines, corresponding to the minimum comprehensive safety factor Fs, min and the most unfavorable instability fai.