Abstract: By using conventional observation data,S-band dual polarization radar data,wind profile radar data and ERA5 (0.25o×0.25o) reanalysis data etc,the environmental conditions,structure and development mechanism ,causes of extreme strong winds of two squall line are compared and analyzed which continous affecting Zhejiang under the same weather background,the results show that:(1)Two squall lines occurrs under the background of strong circulation development in the high-altitude southern trough, low-level vortex shear and surface low-pressure trough. Squall line 1 is triggered in western Hunan, mainly affecting the central region of eastern Zhejiang. The convective cells are arranged in a straight line and distribute in a northeast-southwest band. Squall Line 2 is triggered in western Jiangxi, mainly affecting the central region of western Zhejiang. The convective cells are arranged in an arch shape and distribute in a north-south direction.(2)The main development mechanism of squall line 1 is the strong dry intrusion in the upper atmosphere, coupled with the upward transport of high-energy and high humidity air in the lower atmosphere, which enhances convective instability. At the same time, strong vertical wind shear in the high and low air, strong convergence on the ground, and large convective effective potential energy provide lifting and unstable energy conditions for the squall line. The thunderstorm high and cold pool are relatively weak, and the self-development mechanism of the squall line is weak.The main development mechanism of squall line 2 is the strong thunderstorm high and cold pool. The self development mechanism of squall line is strong, and the direction of low-level vertical wind shear is almost perpendicular to the direction of the squall line. The low-level vertical airflow continues to rise, and the squall line develops. At the same time, the balance between the cold pool and low-level vertical wind shear is maintained, which is conducive to maintaining the life of the squall line.(3)The extreme wind of squall Line 1 occurs during the weakening stage,and the propagation of cold pool occurs earlier than the generation of extreme wind. The drag effect of precipitation particles and the downward transfer of momentum are both contributing factors to the generation of extreme winds.The extreme wind of squall Line 2 occurs during the mature stage,and the propagation of cold pool occurs later than the generation of extreme wind. The extreme wind is mainly caused by strong downdrafts.