The onedimensional timeindependent cumulus model is used to compute the hail size.By means of the sounding data at 07 o'clock as input,the altitude and temperature of the cumulus top,the altitude of the cumulus base,the altitude of 0℃ level in cumulus,the distributions of temperature and vertical velocity of upward current and the water content in cumulus can be given.We calculate the growing and melting of hail and forecast the maximum hail size as it falling on the ground.Comparison of forecast and measurement of the maximum hail size make us satisfied.It indicates that using this model to forecast the maximum hailstone size is feasible.This provides a method of hail forecasting.