Using the conventional sounding observation data, minutely automatic weather station (AWS) data, wind profile radar data, and Doppler weather radar, this paper analyzes the weather characteristics of a regional extreme gale in the northern Zhejiang and Hangzhou Bay coastal areas from the evening to the night of 30 April 2021, focusing on the evolution and mesoscale features of the severe squall line system after its moving into Hangzhou Bay. The analysis results show that this severe squall line system developed under a typical multi-scale interaction background of anomalously deep northeast cold vortex at relatively high altitude. The combination of the mid-level northwest jet and surface warm low pressure induced an enhancement of the local convection storms behind the squall line, resulting in thunderstorm and gale. After the convection cell passed through the Hangzhou Bay, its intensity was enhanced significantly. The warm-moist southwest air flew into the front of the gust front, the cold pool developed stronger in the rear of the gust front, barometric pressure surged with the land surface 〖JP2〗environmental wind field and the thermo-〖JP〗dynamic conditions of water surface of Hangzhou Bay. All these factors triggered unstable energy and enhanced the development of cells. The intensity of subsidence divergence outflow developed strongly after the cell’s crossing the Hangzhou Bay, and it conveyed the momentum in the mid-high layers to the ground rapidly, which generated the enhancement effect of extreme winds in the south of Hangzhou Bay significantly. The lower friction on the water surface of Hangzhou Bay and the special topography of bellmouth were the important causes for the occurrence of extreme gale. At the same time, minute-by-minute temperature change was about 7-10 minutes earlier than the occurrence time of extreme gale. This finding has certain directive significance for local extreme wind monitoring and warning.