Abstract: Herein, numerical simulations of a hailstorm that took place in Guiyang, Guizhou Province, on May 19, 2020, were conducted based on the Weather Research and Forecasting model version 4.2. It was found that the numerical simulations generally reproduced the moving path of the hailstorm in Guiyang. By analyzing the dynamic, thermodynamic, and microphysical structures of the hailstorm during the different developing stages and diagnosing the tendencies of the hail mixing ratio, the microphysical mechanism of hail formation was examined, and a conceptual model was developed. It was concluded that (1) the simulated hailstorm in the mature stage was characterized by a super-cell storm, existing free-echo, and overhang region. The updrafts reached up to 12 km with a maximum vertical velocity of about 20 m s⁻¹ and then flew out eastward because of the westerly wind in the upper troposphere, forming wide anvil clouds. Strong downdrafts were found at lower levels in the rear of the updraft region. (2) Hail embryos were formed through the collection of raindrops by cloud ice, occurring at 6–7 km, and then grew by collecting supercooled cloud droplets and raindrops. The main sink term of hail was melting to rain. (3) During the mature stage of the hailstorm, the collecting rates of supercooled cloud droplets by hail, which mainly occurred at the upper levels of the strong updrafts (6–10 km), increased significantly and produced large mass content of hail with a maximum at 11 g kg⁻¹ in the region of overhang echo and the upper levels of updrafts. (4) During the dissipating stage of the hailstorm, the vertical velocity decreased, and the anvil clouds in the upper troposphere expanded. The net production rate of hail decreased and resulted in a decrease in the hail mixing ratio, while the melting rate of hail into rainwater increased and produced a large amount of rainwater near the ground.