During the construction of karst tunnel, it is difficult to avoid approaching the cavern, even high pressure water cavern. Water inrush damage very easily causes safety accidents and would have irreversible impact on the tunnel. The study on damage mode is conducive to solving problems related to karst tunnel safety and has certain significance for the safety of route selection. In this study, the physical and mechanical parameters of micro-discrete particles are calibrated and verified by a three-dimensional discrete-continuous coupling numerical technology, and the important process of rock-burst collapse prevention between the underlying solution cavity and the tunnel invert under water pressure is simulated. The results show that the failure modes of outburst prevention rock mass are divided into three types: shear failure mode, bending failure mode, and composite failure mode. The bending failure mode indicates that the tensile cracks in the middle and both ends of the outburst prevention rock mass are in the form of penetration; the shear failure mode shows that the cracks at both ends of the outburst prevention rock mass are in the shear state; while the composite failure mode has the common characteristics of both. The fracture development rules caused by the three failure modes are similar and can be divided into three stages: initial development, rapid development, and gentle development. At the stage of initial development, the number of cracks in the rock body is small; the number of cracks in the rock mass maintaining water pressure and preventing outburst suddenly increases and enters the stage of rapid development; after that, the crack in the outburst prevention rock mass connect and then enter the stage of gentle development, ultimately, leading to the overall collapse of the outburst prevention rock mass. Thus, this study indicates that water inrush damage is a gradual process in karst tunnels, but it has an irreversible impact on the overall safety of karst tunnels.