Abstract: In the general parameterization models of hailcloud the value of intercept of hailstone size-distribution was defined by a constant and the melting effect on the decrease of hailstone number had not been considered, so that the feature of hail microstructure at below 0℃ level was distorted. To improve the model we have developed a numerical model of a hail-bearing cumulus which is one dimensional and time-dependent, with a biparameter size-distribution associated with the melting of hailstones. The computed results of this model show that the feature of hail-microstructure below 0℃ level is more appropriate and that the air humitity can influence cloud's updraft distribution profile which is important for the formation of hail or rain.