Based on the synoptic situation and the wind profiler data,the detailed physical analysis of a tornado,which occurred on 6 September 1999,was given.Accumulating with westerly trough,the momentum of Tropical-Storm-9909′s remains increased suddenly and a mesoscale convective complex (MCC) formed.Four tornadoes developed in this MCC.It is found that there are apparently inversion structure and a 360-degree backing of wind direction in low level warm column at the mature stage of tornado.The air is very unstable with an index of SI below -6℃.