Abstract: According to the acquired research,the characteristics of the saddle pattern disposition of large-scale circulation background and the physical process of the southwest vortex during the occurring period of heavy rainfall in the eastern Sichuan basin from 1 September to 6 September 2004 are analyzed and validated from large-scale circulations,water vapor transport and temperature advection by using the observed data and are diagnosed from the vertical and horizontal components of moist potential vorticity i.e.Pm1 and Pm2 respectively.The analyzed results show as follows:(1) During the period of heavy rainfall,there still was a distinct saddle pattern disposition of large-scale circulations.(2) Under the background of the saddle pattern disposition of large-scale circulations,strong southwesterly flow could directly enter into the basin around the east of the Tibetan Plateau,and weak southwesterly flow also could enter into the eastern Sichuan basin over the Yunnan-Guizhou Plateau,which formed a convergent zone of northward jet stream in the east of the Sichuan basin due to the stopping effect of topography and the effect of the westward shift of the western Pacific subtropical high.Moreover,since these two flows also transported a great amount of water vapor,a convergent area of warm and wet air with high temperature and humidity was caused in the northeast of the Sichuan basin.(3) It can be shown from the diagnostics of the vertical and horizontal components of moist potential vorticity that during the occurring period of heavy rainfall,since dry air continuously entered into the upper level over the Sichuan basin,the strong instability of vertical convection,i.e.,Pm1<0,was caused over the basin and developed toward the northeast of the basin,which made the continuous intensification of cyclonic vorticity,i.e.Pm2,a low-vortex strongly developed over the area.Moreover,the horizontal gradient of equivalent potential temperature θe of warm and wet air in the low level over the Sichuan basin,which can caused Pm2>0,also played an important role in the development of the southwest vortex and the occurrence of heavy rainfall,and the center of positive Pm2 was also in good agreement with the area of heavy rainfall.This shows that heavy rainfall used to occur in the instable region of vertical convection of air with high temperature and humidity.(4) The background of the saddle pattern disposition of large-scale circulations,the water vapor convergent zone of the Bay of Bangal and South China Sea,the dry and cold air in the north of Sichuan,and the development of the southwest vortex were the same characteristics of the heavy rainfall in the east of Sichuan.