Abstract: A rare and severe hailstorm occurred on 16 August 2019 in Zhucheng, Shandong province is analyzed to investigate the structure of the hailstorm and the formation mechanism of large hailstones. Radar observations from Weifang CINRAD-SA (China New Generation Radar, S-band) and Qingdao S-band dual polarization Doppler weather radar, soundings and surface observations collected at various meteorological stations, and hail information obtained from field surveys are analyzed to reveal the weather background, the storm disaster situation, the evolution of radar echoes, the structure of the hailstorm and the formation mechanism of large hailstones. The results show that under the influence of a cold vortex system, the low levels are warm and wet while the upper levels are dry and cold over the central mountainous area and the southeast of Shandong province. Strong vertical wind shear of around 30.3 m/s from surface to 6 km altitude is conducive to the development of strong hailstorm. The hailstorm developed rapidly and went through five stages: Initiation, leap increase, brewing, hail fall and extinction. In the initiation stage, mesocyclone and Bounded Weak Echo Region (BWER) could be observed, which continuously strengthened and maintained for a long time. In the hail fall stage, the hailstorm showed characteristic structures such as BWER–overhang echo–echo wall and S-type horizontal circulation field. Furthermore, the BWER was associated with the configuration of the rotating updraft and the "0 line" structure where the horizontal velocity was zero. The "0 line" crossed the overhang echo and the strong echo region above the BWER and pointed to the top of the clouds with a specific hail forming function. In the heavy hail fall stage, the horizontal reflectivity factor (Z_H) in the echo wall to the north of BWER and the strong echo region in the upper hail clouds was greater than 60 dBz, and differential reflectivity (Z_DR) values were mostly between −1—0 dB, indicating it was the accumulation area of hailstones. Based on the shape of the radar echo, radial velocity and three-dimensional circulation field in the mature stage of the hailstorm, the framework of the main updraft in the hailstorm and the schematic diagram of the "0 line" structure with hailstorm forming function are revealed in the present study. The results are helpful for better understanding possible mechanisms of the "0 line" structure in the growth of large hailstorms.