Abstract: Based on the plastic strain distribution obtained from elasto-plasticity FE simulation, the influence of seepage on the failure mechanism is studied through comparison of mechanism under dry or drained conditions. A new failure mechanism under seepage conditions is proposed by improving the mechanism under dry or drained conditions. The face stability of shield tunnel is studied, and the upper-bound solution of limit support pressure is calculated. The proposed method is verified through case studies and applied to the analysis of face stability for Qianjiang tunnel. The analysis shows that the seepage has influence on the failure mechanism of tunnel face, and that if the ground water table is lower than the ground, the influence will be the same and independent of the tunnel diameter and the friction angle. A large part of support pressure is used to equilibrate the seepage force, and the total support pressure almost has linear relation with the water level.