Based on surface and radiosonde observation, NCEP/NCAR reanalysis data and weather radar data, a severe convective weather event with thunderstorm, hail and squall line on March 4, 2018 in Guangxi, Hunan and Jiangxi province was analyzed. The results show that the coupling of high-level jet and low-level jet streams caused strong convergence and upward movement between Guangxi and Jiangxi. The 700hPa warm center + shear line, 850hPa warm ridge + shear line, and ground warm low level inverted trough + convergence line + cold front triggered and organized the severe convective weather event. Extreme temperature difference and water vapor distribution form abnormal thermodynamic instability and potential instability. The dynamic instability caused by strong vertical wind shear in the middle and lower layers plays an important role in the organized development of squall lines. The bow-shaped squall lines have a large inclined angle with the wind direction and move northeast, steered by low-level jet stream with a steady speed of 60-120km/h. With strong low-level wind shear，the cold pool can trigger the front air to produce strong upward movement, which is favorable for forming a new squall line. 10-30 minutes before the occurrence of surface strong wind, the height of strong wind in the middle layer decreased significantly.