Abstract: Focused on the water inrush mechanism of fissure water-resistant rock mass ahead of karst tunnel,the water pressure and instability criterion for tensile-shear failure of rock bridges in fissure water-resistant were derived,and the failure mechanisms under tunnel excavation,rainfall,and their coupled effects were systematically analyzed.The three-dimensional discrete element numerical simulation method capable of accounting for rock structural characteristics was employed to analyze the catastrophic evolution laws during under various disturbance conditions.The results indicate that as the tunnel approaches the karst cavity,the tunnel face displacement transitions from being dominated by excavation unloading to a coupled unloading-seepage effect,with both displacement and water pressure increasing continuously.The displacement at the center of the face is significantly larger than at the periphery and exhibits abrupt changes.Rainfall leads to a continuous increase in face displacement,with a sharp acceleration in the growth rate immediately prior to critical failure.The coupled effect of excavation and rainfall further intensifies the displacement growth rate,while the overall trends remain similar to those under individual effect,but the response is more severe.The research outcomes can provide a theoretical basis for the prevention and control of water inrush and emergency response in karst tunnels.