Abstract: A supercell storm has a vertical circulation composed of a strong updraft and a downdraft. It is easier to understand the formation of the strong updraft in the case of an unstable stratification but it is more complex in describing the process of the downdraft inside the cloud. For this reason, two factors should be considered, one is that the ambient dry-cold airflow breaks into the rearpart of the cloud rather than slowly mixes by entrainment and turbulent processes; the other factor is the transport of liquid hydrometeors from the updraft region to the downdraft region. A one-dimensional time-dependent numerical model was designed which has two branches. The first branch is used to describe the cloud-precipitation developing process and to provide the liquid hydrometeors; the other one is used to simulate the development of the dowadraft. The calculation indicated that the dry-cold ambient airflow breaking into the rearpart of cloud plays a critical role in the occurrence of the downdraft while the transport of liquid hydrometeors satisfies the requirement for the persistance of moist adiabatic process otherwise the downdraft will be damped. In addition, the intensity of downdraft varies with the strength of the ambient air injection, with the amount of liquid hydrometeor transport, and with the microstructure of hydrometeors as well.