A severe convective storm accompanied with strong wind,heavy rain and hail on 23 August 2001 in Beijing was simulated by a three-dimensional cloud model,and was compared with the observed data.It showed that this model could simulate the characteristics of the severe storm such as life cycle,rainfall distribution and hail size,and also could simulate the strong downdraft and wind shear(downburst).The mechanism of the downburst formation was analyzied based on the cloud microphysics of the simulated storm and it was found that this downburst was primarily promoted by negative buoyancy which is produced by the trailing of hail in upair,and also enhanced by cooling resulted from hail melting and rain evaporation.And the trailing and melting of hail played a decisive role in producing the downburst,while the gale was caused by the severe downburst.