Researching pollutant gas cloud infrared spectrum simulation,and using simulate spectra to research how to recognize pollutant gas,were current common technique,but the realtime simulation had not been developed yet.In the article a method of pollutant gas cloud spectrum realtime simulation based on Gauss diffusion mode was founded. With this method,the realtime simulation was processed.The simulated spectra were compared with the measured data,which got good results.Gauss diffusion mode was compared with equality mode,pointed the disadvantage of equality mode.The method of pollutant gas cloud spectrum realtime simulation was very important to the recognition of the pollutant gas leakage.