A largescale severe convection weather event occurred in South China and Jiangnan Region during 4-5 March 2018. Many places were hit by thunderstorm, hail and shorttime intense precipitation, and, especially in Jiangxi Province, severe gale disaster was caused by squall line. Based on the characteristics of atmospheric circulation and radar echo evolution, the process can be divided into three stages: initial stage, developing stage and weakening stage. In the initial stage, the low pressure trough caused by southwest jet before westerly trough provided largescale trigger conditions for the severe convection; in the developing stage, squall line occurred before the trough of warm zone, characterized by extremely strong winds; after into the night, the cold front invaded from the north, making the low pressure trough and severe convection weakening. The diagnosis of ambient field and convective parameters shows that the high lowlevel temperature and humidity, dry and cold middlelevel air, and large temperature lapse rate are conducive to the generation of thunderstorm and extreme gale. The analysis of longtime series of soundings at Nanchang Station shows that the climate states of temperature and humidity are abnormal. Compared with the same period in history, the bottom layer was obviously warmer and wetter, and the middle layer was drier, which is beneficial to the occurrence of extreme convection. Analyzing the multisource observation data and radar data, we summarize the characteristics and causes of the Jiangxi squall line event during this process as follows. (1) Under the combined action of steering flow and forward propagation, the squall line moved very fast. (2) Surface pressure field presented strong thunderstorm high after squall front and the leading low caused strong density flow, which is conducive to extensive straightline gale at the mature stage of squall line. (3) The comparisons of the northsouth structural difference of the squallline bow echoes show that precipitation particles fallen in the midlevel dry rear inflow and the sharp cooling formed the strong sinking motion (downburst), which is a major cause for the extreme winds. The enhancement of thunderstorm high pressure caused by the downdraft in the stratus clouds and the downward momentum transportation both enhanced the thunderstorm gale.