To reduce water inrush disasters in karst tunnels, the formation mechanism of water inrush disasters in karst tunnels and three positional relationships between karst caverns and tunnels (the cavity is orthogonal, oblique, and parallel to the axis of the tunnel) are investigated. Using the corresponding orthogonal mode with the largest requirement for the thickness of the waterinsulating rock disk (the highest risk level) as an example, the numerical simulation method is used to examine the effect of the tunnel surrounding rock grade, cavern water pressure, and cavern size on the design thickness of the waterinsulating rock disk. The minimum thicknesses of the waterinsulating rock disk of the karst tunnel under various working conditions are then compared to study the controlling factors of the thickness of the waterinsulating rock disk. Furthermore, the optimal design thickness expression of the waterinsulating rock disk with and without the consideration of the karst cavity volume is determined. Finally, results are compared with the situation of DK354+460 cavern in Yuanliangshan tunnel, and the results show that the safety thickness value calculated using the formula herein is relatively reasonable, which can serve as a reference for realizing the scientific prediction of the safe thickness of waterinsulating rock disks in tunnels in karst areas.