Using datasets of conventional observations, regional automatic weather station (AWS), Doppler weather radar, wind profile radar and GFS reanalysis with 0.5°×0.5° resolution, we have conducted analysis on the mesoscale characteristics and cause of an extreme thunderstorm gale event in Jiangxi on 4 March 2018, and discussed its extremeness. The results indicate that (1) this event is caused by a strong squall line, and pre-squall depression, squall front, rear-flow depression as well as the thunderstorm high are very clear in it. (2) The squall line developed intensely after it entered Jiangxi under a favorable environment, including the unstable stratification of dry and cold air in the high level and warm and humid air in the low level which was created by strong low-level warm and wet air advection, middle-level dry air, ground warming and low-level wetting, the strong lifting conditions formed by middle-level trough moving eastward, multi-layer jet superposition and convergence line in ground level, and the strong vertical wind shear in the mid- and low- level prolonging its life. (3) The increased angle between the squall line and the steering flow makes it move speedily, and both the flat topography around the Poyang Lake and the rich water vapor enhanced surface gale. (4) Extremely unusual values of physical variables at Nanchang station and their differences with respect to corresponding years of dekad norms exceeding 2σ (σ is root mean square), including surface depression, downdraft convective available potential energy (DCAPE), the temperature difference (T850-500) between 850 hPa and 500 hPa and the vertical wind shear from 925 hPa to 1000 hPa etc., show a good indication to the potential prediction of extreme thunderstorm gale. In addition, high wind zone below 1 km in wind-profiling radar products appears 30 min ahead, and a substantial jump of refractivity structure constant (Cn2) in the upper-level appears 10-20 min ahead. Both can be used as reference in the early warning of thunderstorm gale.