Abstract: Using the 3-D hail cloud numerical model developed by author,a hail cloud occurred in Xunyi area,Shanxi province on July 8,1997 is simulated to analyze mechanismof hail formation and hail suppression with seeding.The results show that embryoes of 97% of hailare frozen drops.The seeding experiments with AgI for heig hts show that if the seeding is made before hail formation,the o ptimum seeding position is located in the maximumupdraft area and its center,i.e.,AgI is seeded in the zone with high water content to be coordinated with maximum zone of the updraft.The seeding make concentration of graupe land frozen drop increase and their average mass/size decrease, so that proportion of conversion from graupeland frozen drop into hail descend greatly and mass and concentration of hailstone are all decrease to achieve our purpose for hail suppression.