Using the data of conventional observation, FY-2F satellite, S-band dual-polarization radar and ERA5 reanalysis, the circulation situations, environmental parameters, characteristics of dual-polarization parameters and triggering mechanisms of the extremely strong convection weather on 17 May 2020 in Shandong are analyzed. The results are shown below. (1) This extremely strong convective weather occurs under the background of the northeast cold vortex with the low-level jets and convective available potential energy significantly enhanced at night, the precipitable water and vertical wind shear both exceeding the threshold values in Shandong and suitable height of the 0 ℃ layer and melting level, all of which provide favorable environmental conditions for the extremely strong convection. Hails are located in the superimposed area of dryline, strong upward movement and water vapor convergence. (2) The supercell storm formed in Weifang led to the occurrence of the large hails in Changyi, Weifang and the exceptionally large hails in Pingdu, Qingdao. The rapid decrease in the vertically integrated liquid water content during the mature phase of the storm, as well as the descent of the storm top and the centroid of the storm cell, has indicative significance for the hail prediction. (3) The supercell storms that resulted in the different hail sizes between Changyi and Pingdu have distinct dual-polarization characteristics. Both have a large ZH corresponding to a small ZDR, Cc and KDP, as well as strong hail attenuation and beam non-uniform filling characteristics, ZDR arcs in the inflow region and near the mesocyclone, and higher ZDR columns in the updraft area. In comparison with the hails in Changyi, the hails in Pingdu have a larger ZH, but smaller minima of ZDR and Cc. (4) This strong convective weather process is primarily triggered by the dryline, with convective storms developing violently where the surface convergence line meets the dryline.