The influence of middlelatitude cold air and topographic forcing on the abnormally severe rainfall event induced by a typhoon depression in Jiangxi Province on 2 to 4 September 2005 is analyzed by using the conventional and intensive observational data, the new atmospheric probing data (including automatic weather station data, lightning localization data, radar images, satellite images), and NCEP reanalysis data. The results show that under the background condition of the subtropical high confronting the land high in the Western China, the middlelatitude weak cold air invading the northern part of the typhoon depression from below unceasingly at the 850 hPa level led to the strengthening of the NE current on its north side; with the main cold air entering the area south of Jiangsu and Anhui provinces from the middle latitude, its barotropic structure transformed slowly into the baroclinic state of “warm in the east and cold west”, and the horizontal distribution of the total energy also changed from spiral vortex to NESW structure, which confirms that the supplement of baroclinic disturbance energy is advantageous to the longtime maintenance of the typhoon low; the analysis of the barotropic part of wet potential vorticity indicates that it changed from the convective stability to the convective instability during the disappearing period of the typhoon low; the strengthening of the energy front in the planet boundarylayer triggered the generation of the mesoβ scale convection system, which produced the severe thunderstorm weather; the severe precipitation mainly distributed on the windward slope east of the Dabie and Jiuling mountains, which demonstrates that the orographic lifting and convergence effect contributed greatly to precipitation enhancement.