In this study, a hailstorm that occurred in Guiyang, Guizhou province on 19 May 2020 was numerically simulated using the Weather Research and Forecasting (WRF) model version 4.2. It was revealed that the numerical simulation generally reproduced the moving path of the hailstorm in Guiyang. By analyzing the dynamic, thermodynamic and microphysical structure of the hailstorm during the different developing stages and diagnosing the tendencies of hail mixing ratio, the microphysical mechanism of hail formation was studied, and a conceptual model was given. The main conclusions were as follows: (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 flow out eastward as a result of the westerly wind in the upper troposphere, forming wide anvil clouds. Strong downdrafts existed at lower levels in the rear of the updraft region. (2) Hail embryos were formed through the collection of raindrops by cloud ice (Piacr), occurring at 6-7 km, and then grew up by collecting supercooled cloud droplets and raindrops (Psacwg, Pracg). The main sink term of hail was melting to rain (Pgmlt). (3) During the mature stage of the hailstorm, Psacwg, which mainly happened at upper levels of the strong updrafts (6-10 km), increased significantly and produced large mass content of hail with a maximum value of 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 upper troposphere expanded. The net production rate of hail decreased and led to a reduction of hail mixing ratio, while the conversion rate of Pgmlt increased, and produced large amount of rain water near the ground.