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Based on the observation data from the automatic rainfall station and NCEP/NCAR reanalysis data, circulation analysis and diagnosis analysis of potential vorticity in a heavy rainfall process in September 2017 in Qujing were conducted. The results show that the heavy rainfall was mainly affected by the convergence zone between the two high pressures and shear line system, the precipitation area developed from the west to the southeast of Qujing, and the heavy rainfall area was concentrated in the central area. The southerly airflow in the middle and lower layers of the troposphere provided favorable water vapor conditions for heavy precipitation. The precipitation was characterized by high intensity, strong suddenness and small time scale. The heavy precipitation was concentrated from 04:00 to 06:00 on September 6. The high- and low-value centers of dry potential vorticity PV at 500 hPa were indicative of the activities of cold and warm air in the process of heavy rainfall, and representative of the development of the convergence zone and the activities of shear line. That is to say, the high-value zone of dry potential vorticity PV at 500 hPa was strengthened to penetrate into the center of Yunnan, leading the northerly airflow to the convergence zone. The deep southerly airflow in the relatively low-value zone of PV was uplifted to the north, converging the cold and warm air at the front of the convergence zone, resulting in heavy precipitation in Qujing. The high-value zone of wet potential vorticity MPV at 700 hPa can also indicate the characteristics of unstable warm and humid air flow activity. MPV1 <0, and MPV2 >0, which easily caused the unstable energy release of wet convection and formed heavy precipitation. [ABSTRACT FROM AUTHOR]