For the toluene's characteristics of flammability, explosiveness and volatile, this paper carried out transient numerical simulation for toluene vapor diffusion in limited space. Emphatically analyzed the concentration distribution in space and the concentration change with time. The simulation result shows that in the lower plane, toluene vapor diffuse from the center to surroundings under the action of diffusion and deposition. The concentration of each position in space increases with time but the increasing rate is non-constant. So toluene vapor accumulates layer by layer during the diffusion.