Based on the routine observational data, SWAN networking charts, radar data of Changsha Station, intensive automatic weather station data and 1°×1° NCEP 6 h reanalysis data, the circulation pattern of the squall line in front of a typhoon on 18 July 2014 in Hunan and Jiangxi Province and the Doppler radar characteristics are analyzed. The results show that: the preferable vapor conditions in the lower atmosphere, the unstable thermal stratification, the high lapse rate of temperature gradient, and high CAPE provided favorable potential conditions for the development of the squall line before generation; the inverted typhoon trough was the main influential system of the squall lines, and the front and the surface convergence line triggered the squall line; the bowshaped echo, midaltitude radial convergence (MARC), highspeed value zones, gust fronts, etc. appeared during the westward and northward movement of the squall line. The squall line in front of typhoons are different from westerly ones: at the low levels the vertical wind shear is mainly dominated by wind velocity differences; the cold air invasion is not as obvious as in the middle and upper levels; there is no evident thunderstorm high at the mature stage, but distinct positive pressure variation is detected; the squall line process happened in warm and humid environmental conditions, with influx of southeast jet stream on its rear side, and its evaporation of rainfall was weaker than that of westerly squall lines. The characteristics of MARC on the velocity diagram were apparent, yet without massive differences between positive and negative velocities (15 to 27 m/s), which are the reasons why the squall line went through Hunan Province without bringing severe disasters.