Abstract:A squall line occurred in Sichuan Basin on 11 April 2022, causing extreme thunderstorm gale (37.4 m·s-1). Due to the poor understanding of the rapid development mechanism of the squall line, there were large deviations in the forecast of short-time severe rainfall and gale during the squall line process. Based on multi-source observation data and ERA5 reanalysis data, the circulation background and echo evolution as well as the maintenance and extinction of the squall line are analyzed. The results are as follows. The squall line occurred in the mutual coupling area of upper trough at 500 hPa and the southerly jet at 700 hPa. Upper-level divergence and low-level convergence were significantly enhanced before the squall line formed, which provided dynamic conditions for the development of the storm. In the south-central part of Sichuan Basin, the convective available potential energy (CAPE) was greater than 1000 J·kg-1, the deep and shallow vertical wind shears were greater than 15 m·s-1, the height of wet bulb temperature 0℃ was 3.8 km， the height of -20℃ was 7 km, the vertical temperature lapse rate was 6.88℃·km-1 and a “bell mouth” existed in temperature humidity profile below lifting condensation level. Especially, the vertical wind shear, mid-level dry layer and downdraft CAPE approached or exceeded the extreme values, which provided favorable environmental conditions for the merging and development of the squall line and the occurrence of severe thunderstorms and gale. Convections in the northern segment of the squall line were triggered, developing under the combination influence of the terrain of Longquan Mountains, cold pool and surface convergence line. In the south section, convections were uplifted by the dynamic action of the lower level. Due to the 700 hPa jet stream, surface cold pool and convergence line, two sections of linear convection were intensified and merged into a north-south squall line system with super-cell storms and bow echoes when moving eastward to the high energy and humidity areas in central basin. Super-cells before the formation of squall line and the echoes of the mature squall line both had overhang echoes, weak echo zones, rear inflow and mid-level radial convergence. Downdraft formed by the rear inflow and dry curl in the middle troposphere made the supercell storm and cold pool move faster than other parts. Then, a bow echo was formed. Because of the gale of downburst and precipitation, propagation velocity of the cold pool was significantly stronger than the 0－3 km vertical wind shear, which was an important cause for the rapid extinction the squall line.