Using conventional observation data, S-band dual-polarization radar data, ERA5 reanalysis data, and brightness temperature of satellites FY2F, the circulation situation, environmental parameters, dryline triggering mechanism of the extreme strong hailstorm, as well as the characteristics of the dual-polarization parameter of the strong hailstorm with different sizes on May 17 2020 in Shandong are analyzed. The results show: 1) This extreme strong convective weather occurred under the background of the northeast cold vortex, with significant enhancement of low-level jets and CAPE at night, PWV and vertical wind shear both exceeding the threshold values in Shandong, the hail area was located in the energy front, strong upward movement, and water vapor convergence area, the suitable height of the 0°C layer and the melting level provided favorable environmental conditions for the production of large and super large hails. 2) The supercell storm formed in Weifang led to the occurrence of large hail in Changyi and exceptionally large hail in Pingdu. The rapid decrease in the vertical integral of liquid water content during the mature phase of the storm, as well as the descent of the storm top and the center of mass of the storm cell, have good indicative significance for the descent of hail. 3) The difference in hail size between Changyi and Pingdu resulted in differences in the dual-polarization characteristics of the storm. Both have a large ZH corresponding to a small ZDR, CC, and KDP, all showing strong hail attenuation and beam non-uniform filling characteristics; there are ZDR arcs in the inflow region and near the mesocyclone, and a higher ZDR column in the updraft area; and the larger size of hail in Pingdu, with the ZH larger than that in Changyi, and the minima of ZDR and CC smaller than that in Changyi.4) This strong convective weather process was primarily triggered by the dryline, with convective storms developing violently where the surface convergence line meets the dryline.