Abstract: By using Doppler weather radar 3830/CC and NCEP 1°×1° reanalysis data, a supercell hailstorm process in Southwest Yunnan was analyzed under the influence of the south branch trough. The results showed that the mesoscale radial wind field structures, such as a stable and lasting mesocyclone for more than 2 hours, the low-layer convergence and high-layer divergence, and the wind speeds increasing with heights, were conducive to the low-layer warm and humid air’s converging into the cloud and tilting and rotating up, leading to the formation of a weak echo area (WER) and a hook echo in the low layer, a mid-high layer echo overhang, and a V notch formed by the invasion of the cold advection on the north side. All of these combined ensured the supercell hailstorm lasting more than 4 hours, and the hail particles were strongly scattered to form sidelobe echoes. Radial wind convergence gradually enhanced and extended to a height of 8 km. Echo parameters, such as intensity, top height, 50 dBz height, −20 ℃ layer intensity, vertical integrated liquid water content (VIL), increased fluctuatingly. Especially the mesocyclone was very conducive to the development of the supercell and the growth of hail particles. The previous five characteristic parameters had their maximum values within 26 minutes after the mesocyclone reached the strong stage (rotation speed ≥10 m·s⁻¹). The confluence of southwestern airflow in front of the south branch trough provided water vapor conditions. The strong vertical wind shear in the middle-low layer provided uplifting dynamic conditions. The mesoscale convergence line in the near-surface created convective trigger conditions, and the low-layer temperature and humidity front area led to the unstable development of convection. Meanwhile, the vertical structure, with the cold upper layer and the warm lower layer, and with moderate humidity at the lower layer, high humidity at the middle layer and low humidity at the upper layer, aggravated convective instability to generate enough updraft, forming a vertical circulation system in which tilted updraft and downdraft coexisted, and causing the continued development of the supercell hailstorm. As a result, hails occurred near the mesoscale convergence line and around the humidity and temperature front in the near-surface layer.