In view of the extreme wind and hail weather in Huaian area from 17∶00 BST to 19∶00 BST on April 30, 2021, by using conventional observation data, ERA5 reanalysis data, Doppler weather radar, dual polarization radar and automatic station minute data, through diagnosis analysis and wind field inversion, the structural characteristics, maintenance mechanism of two supercells and the causes of extreme gales caused by strong convective weather were analyzed. Results show that in the context of the 500-700 hPa northerly jet, the strong convergence center and convergence area on the ground promote the convective cell to become a supercell storm. The extreme gale at Huaian reference station is caused by the downburst event generated by supercell A, which is characterized by the obvious decline of storm centroid height, strongest echo height and mesocyclone bottom height, and the divergent wind field near the surface layer. The maximum reflectivity factor height, storm centroid height and the strongest shear height of mesocyclone of supercell B producing strong hail all reach above the height of wet bulb temperature -20 ℃.The causes of extreme gales include gravity drag and melting evaporative cooling in the falling process of heavy hail and heavy precipitation particles, negative buoyancy and vertical disturbance pressure gradient force generated by low-level strong mesocyclones.