Abstract: Base on Daris at al tunnel failure model and the upper bound theory of plastic limit analysis, a new parameter aequal to 1, governing size of shear zone of tunnel failure model was put forward. The velocity of shear zone and formula of line of discontinuity was applied in using incompressibility condition and velocity continuity condition, deducing energy dissipated on the line of discontinuity, energy dissipated within shear zone and total the work done by the outside force of the model Expression of stability ratios got by applying conversation of energy theory, from the result standpoint, the upper bound solution is smaller than Davis et al the results, thus the upper bound solution to be closer the real solution. From the standpoint of the experimental results comparison, the upper bound solution is located nearby the experimental value, which can agree with experimental results, thus the model is closer the real failure tunnel. The upper bound solution would provide the theory basis of the tunnel support.