To better design the smoke exhaust system for tunnel shafts in high-altitude areas, numerical simulation methods were used to calculate and analyze the effects of environmental pressure and shaft inclination angle on the smoke plug-holing effect during tunnel fires. The temperature fields under different working conditions were obtained using FDS simulations. The plug-holing height was introduced to determine the degree of plug-holing for each working condition. Combined with the degree of plug-holing, the dimensionless Richardson number (Ri') was redefined, and a theoretical calculation model for the plug-holing height was proposed. The critical Ri' number (Ri_c') for the occurrence of plug-holing was derived. The results show that the plug-holing height increases with the increase of the angle and environmental pressure. The Ri' number decreases with the decrease of environmental pressure and increases with the increase of the shaft inclination angle. Under the same environmental pressure, Ri_c' number decreases with the increase of the shaft inclination angle. The relationship between Ri' number and the Ri_c' number can be used to determine whether a complete plug-holing effect occurs.