Abstract: Based on dual-polarization radar, ground data and ERA5 reanalysis data, a large-scale extreme wind event was studied in Hubei province caused by squall line. The results showed that: under the typical thunderstorm temperature and humidity profile(wet downburst) environment, the squall line originating in southwest Henan was significantly enhanced after crossing Tongbai Mountai, and triggered the Derecho event in Hubei Province. The direct reason for the enhancement of the squall line was that several storms on the south side merging into the squall line. Further analysis showed that the key mesoscale systems for the enhancement of the squall line were thin cold outflow of another squall line, the boundary-layer jet raising by the topography and the cold pool outflow of the squall line. The topographic effects included the blocking of cold pool outflow, gap penetration, orographic uplift, which triggered isolated storms and provide a mesoscale ascending environment for storm development. After the squall line crossing the mountain, the extreme wind of Guangshui was mainly caused by the momentum transmission and the divergence of the strong downdraft. In the squall line, the convective cell was composed of graupel or small hail above the melt layer, and many small solid particles were strongly melted into large water droplets or water-covered ice cores near the melt layer. Strong evaporation under the melt layer significantly reduced the diameter of raindrops and the liquid water content. This indicates that the strong melting and evaporation were the main mechanism of the formation of strong downdraft in the storm. The results add to the understanding of the effects of mesoscale topography on storms and the physical processes of the formation of extreme winds.