Navier-Stokes equations are solved to simulate a gymnasium fire. The equations are simplified by weakly compressible low Mach number assumption. Turbulence effect is simulated using Smagorinsky large eddy simulation (LES) model. The mixture fraction combustion model is adopted to simulate the burning process. With the analysis of computed velocity and temperature field, two important phenomena, named door effect and smoke plug-holing, are found to be responsible for the deterioration of smoke exhaust efficiency when natural ventilation or forced ventilation is present. Some explanations are made to elucidate these effects mechanism. An improved design of smoke ventilation system is suggested.