Abstract: Yunnan town automatic precipitation data, short-time heavy rainfall monitoring data, FY2G satellite data, upper air and surface convention data and NCEP reanalyzing data were used to analyze synoptic reasons of a city extreme heavy rainfall event happened in Kunming in the summer of 2020. Research indicated that: Affected by Yunnan-Burma ridge, plateau trough moved eastward and formed plateau vortex in the northeast of Yunnan. The plateau vortex moved southward along convergence area between two highs and then influenced Kunming. Plateau vortex and shear line were main weather systems of the extreme heavy rainfall event, and the heavy precipitation happened near plateau vortex and shear line where vapor converged strongly. Equivalent potential temperature, equivalent potential vortex were used to diagnose instability property. Instability mechanisms of the heavy rainfall event were convective instability and symmetric instability, and symmetric instability maintained the slantwise convection. Boundary convergence line caused surface vertical ascending motion and then meso-scale convective systems produced. Shear line at 700 hPa and vortex at 500 hPa promoted convective systems developing, and vertical ascending motion enhanced. Vertical ascending motion triggered the releasing of instability which leaded to heavy rainfall event. Vortex Producing Parameter positive area may forecast the generating and moving direction of plateau vortex.