<p>During the tunnel construction in karst area, the high pressure and water-rich cavity in front of the tunnel face can cause the geological disaster of water inrush and mud outburst. Therefore, it is a key technical problem to reserve the appropriate safety thickness of rock wall. In this paper, based on the upper limit theorem of limit analysis and Mohr-Coulomb strength failure criterion, a calculation method of safety thickness of rock wall considering the adverse effect of seepage of karst water is proposed. The water inrush criterion of seepage instability on tunnel face is established, and the sensitivity analysis of main parameters is carried out. By defining the safety factor (<i>k</i>) for water inrush and instability of the tunnel face during excavation, the dynamic evolution characteristics of the stability of the water‑resistant rock wall in front of the face of karst tunnels during construction are analyzed. The calculation method proposed in this article was applied to two case histories: the Zhongba Tunnel (in Luzhou city) and Xiema Tunnel (in Chongqing city) respectively, and the calculation results by the proposed method are compared with those obtained without considering the adverse effect of karst water seepage. It is shown that the safety thickness value of water‑resistant rock wall obtained by the calculation method in this paper is in good agreement with the actual engineering reservation value. The research results of this paper are of great significance to improve the predictability and targeted prevention of water inrush disasters in karst tunnels.</p>