A semianalytical solution to determine the minimum safety thickness of the rock resisting water inrush from filling-type karst caves was investigated. Mechanical analyses of the filling materials in karst caves located in a top, bottom, and lateral position to the tunnel were carried out by referring to the principle of the slice method. The mechanical mechanism of rock resisting water inrush was analyzed by adopting the theory of elastic mechanics. The semiquantitative analytical method for calculating the minimum safety thickness of the rock for resisting water inrush was achieved according to the bending strength and shear strength of the rock. The safety-thickness-prediction models of rock for resisting water inrush were applied in practical projects of the Luzhuba tunnel. The accuracy and availability of the proposed method were proven by comparing with the rock stratum thickness–span ratio method. The minimum safety thickness affected by the depth-span ratio of the filling-type karst cave, shear strength indexes of the filling materials, and the depth of the tunnel was analyzed and discussed. The results show that the minimum safety thickness of the rock resisting water inrush (1) decreases 0.02 m for every 2-m increase in cave height and increases 0.01 m for every 2-m increase in cave span such that the influence of the karst cave height is greater than the karst cave span on the minimum safety thickness; (2) decreases gradually with an increase of effective soil cohesion and decreases 0.03 m for every 5° increase in effective internal friction angle of the soil; and (3) increases 0.18 m for every 1-m increase in tunnel depth.