The Weather Research and Forecasting (WRF) model was used to simulate the hailstrom in Beijing on June 25, 2021 in this paper. From the perspective of radar reflectance and precipitation, the numerical simulation basically reproduces the macroscopic characteristics of hail storm. On this basis, the mesoscale thermal, dynamic and microphysical characteristics of hail storm in different development stages are analyzed. Finally, the mass budget and latent heat budget of hail particles and raindrops are analyzed emphatically by outputting the intermediate transformation term of the microphysical process, and the conceptual model diagram of cloud microphysics of hail formation is given. The result of research shows that: the process of hailstorm can be divided into three stages: multi-cell echo,linear convection and squall line. In this hail weather, snow particles are mainly hail embryos, which form hail particles by accretion of rain and cloud water by snow in the middle and upper troposphere, they grow by means of accretion of rain and cloud water by hail after the formation of hail particles. Production rate for accretion of rain by cloud ice or accretion of cloud ice by rain to form hail particles is very low. The airflow flows in at the front lower level of the storm, and the two weak updrafts of the multi-cell stage constantly merge. In the linear convection and squall line stage, the inflow of low layer was weakened, while the inflow of dry and cold air was obvious in the middle and high layer. The air flow rose strongly in the squall line phase and flowed forward at the upper level. When the water vapor is lifted up by the updraft and meets the cold and dry air flowing in from the middle and upper levels, it condenses into cloud water and frozen ice particles, a large amount of latent heat is released, which leads to the enhancement of buoyancy and the strong rise of the air flow in the cloud, lifting up more water vapor to condense into cloud water or ice particles and promoting the formation and accumulation of hail particles. The melting of snow and hail particles absorbs large amounts of latent heat, causing the melting layer to rise. Therefore, a large number of supercooled rain form in the squall line stage, which increase production rate for accretion of rain by snow to form more hail particles. The lower level downdraft will be enhanced by the stronger drag of water material caused by the fall of large hail particles. Downdraft causes evaporation of rain water at lower levels, forming cold pools near the surface by cooling effect , which cooperate with latent heat heating at higher levels to enhance convection. Thus, it positively feeds back to the meridional circulation. The vertical updraft caused by the circulation promotes water vapor in the lower layers to rise above the melt layer and condense into supercooled water and snow particles, while the vertical downdraft promotes snow and hail to melt into rain water and rain water to evaporate below the melt layer, and then positively feeds back to the thermal environment within the cloud. This repetition produced hail and heavy precipitation.