The mechanism of hail formation and seeding effect of hail suppression were studied using a threedimensional hailstorm numerical model with fully elastic primitive equations. The data used were taken from a large-area hailstorm affected by a high altitude cold eddy that occurred in Shandong Province on 24 May 2008. The simulation results showed that the height of the center of supercooled raindrops was below the maximum updraft center in this storm, so the accumulation zone of supercooled rain water was not present, and the maximum content of supercooled rain water was only 4.9 g m^-3. However, the supercooled rain water content was still rich in the hailstorm cloud, which played an important role in hailstone embryo formation. The main hailstone embryos were frozen droplets, with the dominant formation mechanisms being the collision freezing of ice crystals, snowflakes with supercooled rain water and the nucleation of raindrops. Once formed, accretion of rain water and cloud water were the dominant growth mechanisms of frozen droplets. Both the main quality and quantity source mechanism of hail particles was the auto conversion of frozen droplets. After formation, the coalescence of frozen droplets, graupel particles and supercooled rain water were the dominant growth mechanisms of hail particles in the earlier stage, while in the later stage the dominant growth mechanism was the coalescence of supercooled cloud water. Catalytic results showed that hail-related disaster would have been suppressed by more than 50% by seeding 57.5 g AgI, and precipitation would have been enhanced along with the hail suppression by increasing the AgI content. Rainfall precipitation was increased with the solid-phase precipitation and its percentage decreased, especially hailstones, when seeding with 230 g AgI. Artificial ice crystals were produced with the deposition nucleus AgI, and the deposition growth of ice crystal resulted in a reduction of supercooled cloud water content. The number of hailstone embryos, especially frozen droplets, increased considerably after seeding, and these numerous smaller hailstone embryos competing for the lower availability of supercooled cloud water resulted in a reduction of average size and quality of frozen droplets. The quantity of hailstones reduced because the conversion rate of hailstone embryos to hailstones was decreased. Accretion of supercooled cloud and rain water by hailstones was decreased after seeding, resulting in a decrease in hail amount, and the hail was suppressed by seeding with AgI.