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Based on the routine observation, NCEP/NCAR reanalysis data and simulation data, a squall line occurred over the coast of Fujian on 20 June 2014 is analyzed. Results show: (1) This squall line was belonged to the pretrough type, and there was a developing low vortex shear in the lowlevel of the troposphere, with a weak cold air going southward. The lowaltitude southwest jet was not obvious, but there was continuous warm and humid air flow delivered to the northern central area of Fujian. The squall line was formed on the south side of the low vortex. (2) The simulation results show that during the development of the squall line, the lowlevel wind speed on the south side of the system increased continuously and maintained high water vapor transport and unstable energy. At the early stage, the triggering factors of the convection include the cold outflow of the frontsystem, the elevation of the terrain, and the forced uplift of the cold pile caused by sea breeze. At the later stage, the collision between the cold outflow boundary and the sea breeze front boundary strengthened the lowlevel convergence and promoted the development of convection, which was the main reason for the formation of the squall line. (3) The pressure field of the squall line appeared to be a meso high and a wake low in its mature stage. A surface divergence zone slightly lagged the meso high where there was strong surface winds. There were two airflows in the squall line at its mature stage. The forward inflow which was lifted in the convective cloud area was a lowlevel warm and humid airflow in front of the squall line. The rear inflow was a middlelayer dry and cold airflow behind the squall line, forming a sinking motion in the lower layer, which was one of the important reasons for the formation of strong winds on the ground.