This paper enacts ventilation condition and combustibility of materials in a real building with large space basing on a fire investigation report and establishes a FDS physical model. Heat release rate of the fire is calculated and the distribution of temperatures and concentration of carbon monoxide of smoke are discussed. Flashover and backdraft occur in the fire respectively. The authors suggest that the flashover might be a important reason resulting in more people’s death in the fire. The temperatures and concentration of carbon monoxide of smoke are not increased when the backdraft occur

but not declined because the backdraft have an effect on it.