On April 12, 2020, affected by the cold vortex, a large-scale thunderstorm and gale occurred in East China, including an extreme gale exceeding the Beaufort scale 12 in Hangzhou Bay. Before the occurrence of this severe convection process, the high and low levels had consistent northwest air flow, with low water vapor content and weak energy conditions, so it was difficult to predict, thus larger deviations appeared in coastal sea surface wind forecast. Based on conventional observation data and Doppler weather radar and wind profile radar data, combined with ERA5 reanalysis data, this paper analyzes the characteristics of thunderstorm gale in this process and the evolution characteristics of convective system before and after its moving into Hangzhou Bay, and focuses on the possible causes of extreme gale in the northeast of Hangzhou Bay. The research shows that the formation of extremely strong wind in Hangzhou Bay was the result of the joint action of many factors. Hangzhou Bay had certain energy conditions, and the temperature reduction rate of the lower atmosphere was close to the dry adiabatic decline rate, which was conducive to the formation of strong downdraft in the convective system and the surface strong wind caused by subsidence divergence. There was a northwest wind jet in the middle layer. Under the action of convective subsidence, the high momentum in the middle layer was brought to the ground, enhancing the ground wind speed. During the process of the convective system moving towards Hangzhou Bay, the cold pool enhanced, and the friction on the water surface of Hangzhou Bay was smaller than that on land. Therefore, the intense flow in the cold pool spread faster, which was also one of the important reasons for the increasing of wind speed in Hangzhou Bay. The occurrence of gales above Beaufort scale 13 in the northeast of Hangzhou Bay was also related to the uneven distribution of marine thermal and dynamic conditions and the variation of echo shape after the convection moved into the sea.