Abstract:A large-scale extremely severe convective weather named “5·17” severe convection occurred in Shandong Province on 17 May 2020, and the hail coverage was the largest in the resent 10 years. The convective storms were highly organized. The regional supercell clusters and a strong squall line over 500 km in length caused this extremely severe convection. Based on ERA5 reanalysis data, automatic weather station data and Doppler weather radar data, the ambient conditions of this extremely severe convective weather are analyzed. The results show as follows. The cold vortex was located in the key area which was most conducive to the Shandong severe convection. The large-scale weather system forcing was strong, and the unusually strong cold air in the middle troposphere moved southward impacting the previously abnormally warm Shandong Province and resulting in the “5·17” extremely severe convection weather. The anomaly of weather system is more representative of the intensity of dynamic and thermal forcing, and when anomaly level is above 2σ, the extremely severe convection would be caused. When the intensity of the cold vortex weakened during its southward moving, accompanied by increasing anomaly, it may still cause extremely severe convective weather in its southeast quadrant. The strong deep vertical wind shear was conducive to storms organization and development. The long axis of the squall line orientated the same direction as the 0-6 km vertical wind shear vector. The area where new cells were generated, developed and merged was located in the front of the large value center of the wind vector difference. The low-level warm and moist advection continuously transported warm and humid air to Shandong, which was the mechanism of CAPE reconstruction and the main energy source for the long-term maintenance of supercell group and long squall line.