Abstract: A diagnostic study on a severe rainfall occurred over the northeast of Sichuan province in 3 —5 September 2004 is undertaken with the rainfall characteristics, large scale circulations and the meso-scale synoptic systems presented.Results show that wind shear at the east part of Tibetan Plateau and southwest vortex at the lower level in the basin of Sichuan play the key role in the occurrence of the heavy rainfall.The convective activities and the heavy rain mainly occurrson the sharp wind shear which locates over the east of the vortex center.The very strong vertical velocity is detected which is sustainable by two second vertical circulations located respectively at the north and south of the southeast vortex.