During tunnel construction, it is inevitable to encounter bad geological conditions, e.g., waterrich faults, which can cause water and mud inrush disasters. However, such disasters can be avoided if the minimum safe thickness of the waterresistant rock mass can be predicted. Thus, in this paper, based on a tunnel crossing waterrich fault, a calculation model for waterresistant rock mass with punching shear failure mode is proposed, the calculation formula of the minimum safe thickness is obtained, and influencing factors are analyzed. A threedimensional numerical model is established using the FLAC3D software to determine the simulation solution for the minimum safe thickness. Finally, the calculation formula is applied to the Yonglian tunnel to verify its applicability. The results demonstrate the following: (1) The minimum safety thickness increases with increasing fault width, tunnel radius, and water head height, and it decreases with increasing fault inclination, internal friction angle, and waterresistant rock mass cohesion. (2) The simulated and theoretical solutions are in good agreement, and the movement trend of surrounding rock near the tunnel face is consistent with the movement direction of the damaged rock mass assumed by the theoretical model. (3) The calculation formula can accurately predict the minimum safe thickness of a waterresistant rock mass when the tunnel passes through a waterrich fault.