Abstract: To investigate the characteristics of interactions between supercooled water and hail embryos within hailstorms, a multi-cell hailstorm event that occurred on April 18, 2019 in Weining County, Guizhou Province was systematically analyzed, integrating X-band dual-polarization radar observation data, ERA5 reanalysis data, and conventional sounding observations. The results indicate that: (1) Before cell merger, within the −20 to −10 °C layer, supercooled water rapidly froze on ice nuclei surfaces, forming ice crystals and aggregates, which provided the material basis for hail embryo and hailstone formation. (2) During cell merger, within the −10 to 0 °C layer, supercooled water froze slowly on aggregate surfaces, filling surface gaps and bonding with other aggregates, thereby increasing the density and forming high-density graupel with compact structures. Through riming of supercooled water, high-density graupel continuously increased in mass and volume, eventually evolving into hailstones. The rate of change for hail was 18.71 Z/min, while for high-density graupel was −17.43 Z/min. (3) After cell merger, a large amount of hailstones and graupel melted or fragmented during descent. The released moisture was absorbed by smaller particles, leading to a temporary increase in ice crystals and aggregates below the 0 °C level.