Based on the three conditions of formed-hail, a heavy hail event in Zhengzhou area on 19 July 2002 is analysed.The results show that the severe convection weather occurs easily when the water vapor and instability and the outer rising force are all suitable. The severe vertical wind shear and an appropriate height about 0℃ layer and -20℃ layer will be advantageous to the hail augmentation, and the low-level horizontal helicity and potential temperature difference are correlation with the hail area.