Abstract: As tunnel engineering extends into deep zones with high in-situ stress, collapse hazards controlled by the coupling of high in-situ stress and well-developed structural planes have become increasingly frequent. Clarifying how the geometric orientation of structural planes controls the instability mechanism and dynamic evolution of such collapses is therefore an urgent requirement for engineering safety. Taking a typical high-stress jointed-rock tunnel on the newly built Lanzhou–Hezuo Railway as the engineering background, this study integrates field investigations with collapse cases and develops a geomechanical model incorporating dominant structural planes by using the (three-dimensional discrete element method, 3DEC) and a stochastic (discrete fracture network, DFN). The model is used to systematically examine the controlling effect of the spatial orientation of structural planes on collapse mechanisms. The results show that structural-plane orientation controls the location, scale and mode of collapse by altering the boundaries of potential sliding blocks and the stress paths in the surrounding rock. When the strike is parallel to the tunnel axis, the collapse extent increases with the dip angle; when the strike is perpendicular to the tunnel axis, the collapse scale first increases and then decreases with the dip angle, reaching a peak at approximately 30°. At a fixed dip angle, the collapse scale first decreases and then increases as the strike changes, and the most stable condition occurs at a dip direction of 30°. From the perspective of the mechanical mechanism, three stress-structure-controlled collapse modes and their instability indicators are identified: arch-waist bedding-controlled sliding, rock-slab fracture-sliding, and crown-dominated collapse. The evolution of collapse from stress redistribution to overall instability is clarified, and the intrinsic relationship between structural-plane geometric parameters and collapse risk is established. These findings provide a theoretical basis for stability assessment and hazard prevention and control in high-stress jointed-rock tunnels.