Abstract: Under the hydraulic action, the tunnel face in rich water stratum is prone to lose stability and cause safety accidents, so ensuring the stability of the tunnel face is the key to ensure its normal construction. In this paper, the research was based on the dense zone of water rich fissures encountered in the construction of Baoligang tunnel. Based on the fluid structure coupling theory and numerical analysis method, the model was established to study the instability mode of the face. Based on the mechanical model, the seepage failure and instability process of tunnel face were simulated. By simulating the process of seepage failure and instability of the face, the evaluation method of the face stability was proposed. We defined the limit thickness of outburst prevention body to ensure the safety of tunnel construction.