Abstract: By the method of CFD based on K-ε turbulence model,the diffusion of chlorine in the large space of public buildings was simulated. First of all, the model was established with the software of Gambit and then input the software Fluent to get the stimulation of chlorine diffusion cloud. The analysis and comparison of simulation results in different air-condition showed that chlorine, as heavy gas,spreaded close to the ground obviously. When air conditioning was operating,the indoor flow field had a remarkable effect.It caused chlorine to diffuse faster and larger, but the concentration became lower. The study on diffusion rule of chlorine and influencing factors will provides us with important reference for optimizing ventilation design of large space public buildings and the emergency disposal of chemical terrorist attack.