Using the data of precipitation observation, FY2C, realtime radiosonde sounding, and NECP/NCAR reanalysis, the evolution characteristics of key circulation systems, vapor transmission characteristics, moist potential vorticity (MPV), and vertical helicity (VH) of the regional heavy rain event in Sichuan Province and Chongqing from 2 to 5 August 2009 are analyzed diagnostically. The atmospheric circulation background of the heavy rainfall includes the eastward high trough coupled the southwestern low vortex, the dynamic blocking of Typhoon Swan maintaining over Sichuan Province and Chongqing, the southward invasion of cold air joined with the warm and wet vapor from the southwest jet, the formation of a clear cyclonic convergence, leading to the continuous generation and development of smallscale convective systems, and the favorable conditions for water vapor transport maintained in the precipitation process. The MPV was highly indicative for the area of the rainfall in the rainfall process. Due to the largescale descending air above the negative MPV center, the tilt degree of strong updraft, and the locations and intensities of the maximum frontogenesis forcing in the heavy rainfall, the strong precipitation area (Chongqing West) was located in the warm side of the negative center (Sichuan Basin) of MPV. The VH evolution is closely related with the heavy rain. The heavy precipitation occurs (weakens or ends) when VH contours becomes dense (sparse), and the appearance of heavy precipitation accompanies usually the upperlevel (lowerlevel) positive (negative) VH coupling.