Based on conventional observation data, automatic weather station data and CINRAD-SA Doppler radar data, we analyzed the weather background, environmental conditions, development mechanism and the gale that caused for the squall line event on 27 May 2018 in Guangxi. Results show that the cold tongue at 500hPa superimposed with the warm ridge in the lower layer built a strong stratification instability structure, with a high value of CAPE, which provided a favorable environment for the development of the squall line. The mesoscale convergence line located at the coast of Guangxi was the trigger mechanism for the initial convection system that finally leaded to the formation of the squall line system. The warm and moist inflow from the lower layer of the squall line reached the top of the storm and diverged. The cold and dry air inflow from the rear of the middle layer strengthened the downdraft within the squall line. The downdraft outflow formed convergence on the front side of the storm to trigger new convection. The self-organization structure of the squall line was formed by the inflow and outflow stagger. These were the important reasons for maintenance and development of the squall line under the condition of weak vertical wind shear. The main reason of strong wind was the density current caused by the merger and strengthening of surface cold pools.