Abstract: It is a common geological hazard for water inrush when tunneling in a complex geologic area. The water resisting rock strata in front of a tunnel face is an important safety barrier to prevent the water irnush. This paper mainly studies the method of determining the critical safety thickness of the aquifuge in front of the tunnel. Based on the Mohr-Coulomb failure criterion, a shear-damage mechanical analysis model of the water-resistant rock layer is constructed and the corresponding calculation formula for the safety thickness of the water-resistant rock layer is obtained. For deep tunnels, the effect of the structural stress field on the horizontal stress is obvious. Through theoretical analysis and safety angle analysis, it is found that the calculation of the normal stress from the vertical stress is more reasonable. Considering the influence of tunnel excavation on the initial ground stress, the above model is further improved. Combined with the characteristics of water inrush disaster in the Yuelongmen tunnel project, the safety thickness of water-resistant rock stratum is analyzed by using two methods, which are interval non-probabilistic reliability and probability analysis, and the calculated results are compared with the actual field test results. The application results of engineering cases show that the calculation model proposed in this paper is effective. The research result of this paper is not only to complement and perfect the existing knowledge system of the critical water resisting strata thickness, but also to provide auxiliary information for subsequent risk assessment and risk countermeasure formulation.