<p>Water and mud inrush hazards occur frequently during tunnelling or mining, which can seriously threaten personnel security and result in huge financial or environmental disruption. This hazard can be triggered by the hydraulic fracturing of water-resisting rock mass between tunnel face and water-bearing structure. As a result, a large number of methods have provided insight into calculating the minimum safe thickness of water-resisting rock mass, such as theoretical analysis. However, the existing theoretical calculation models have not yet considered the fractal dimension and seismicity for tunnel crossing active fault. Thus, this study corrected stress intensity factor of fractal fracture under seismicity, followed by establishing the initiation and propagation criteria of crack. On this basis, a formula was proposed to determine the minimum safe thickness of water-resisting rock mass. Finally, a typical tunnel crossing active fault was taking as the study case to certify the validity and feasibility of the presented calculation model. Fractures in the water-resisting rock mass of the tunnel show remarkable fractal characteristics under the coupled effects of hydraulic fracturing and seismicity. This is manifested by the dependence of fracture mechanics parameters on fractal dimension, leading to a reduced critical water pressure for crack initiation and propagation. There are two main patterns, including tensile-shear propagation of Mode I-II crack and compressive-shear propagation of Mode II crack. The critical water pressure for the crack initiation and propagation declines due to fractal effect and growing earthquake intensity, ultimately leading to an escalation of the minimum safe thickness of water-resisting rock mass. Moreover, earthquakes with intensities of 9 and 10 can make fracture propagation unrestricted to the thickness of water-resisting rock mass, which means the hydraulic barrier cannot resist water and mud inrush by itself. The results of this study can contribute to the prevention of water and mud inrush hazards for tunnel crossing active fault. The proposed model offers an improved theoretical framework by simultaneously incorporating fractal fracture geometry and seismic dynamic effects—addressing a key limitation of conventional approaches—and can be directly applied to safety design and risk assessment of tunnels traversing active fault zones in seismically active regions..</p>