Abstract: Following the conceptual idea of puff model,a diffusion model that describes pollutants discharged in finite duration of time is set up,with which a complete consistence between the model and the analytic solutions with extremely short duration of release from a transient point source,and a perfect conformity between it and the plume model with sufficiently long duration of release in the form of a continuous point source,are obtained.The discharge of pollution within a limited time is featured by attainment of dynamic equilibrium of concentration at the ground surface beginning from the vicinity of the source shortly after the occurrence of accidents and subcequent release of pollution,and by eventual “disappearence” of pollution that is gradually reduced due to the purifying effect of the atmosphere when the accidents are over,or,there occurs an end to the release.