Abstract: During 17–22 July 2021, a large-scale extreme rainstorm occurred in Henan Province. In this study, ERA5 reanalysis data were used to examine the heavy rainfall process based on the theory of moist potential vorticity and slantwise vorticity development. The forecast capabilities of the high-resolution global model Tianji system and the Integrated Forecast System (IFS) of ECMWF (European Centre for Medium-Range Weather Forecasts) were also investigated. The results show that this event results from the interaction and synergistic influence between the middle-high and low-latitude systems. From 18 to 19 July, the high moist potential vorticity in the middle and high latitudes intruded downward from the middle and upper troposphere along the slantwise moist isentropic surfaces. Together with the cooling and increased relative humidity, this led to the formation of the high vorticity zone in northwestern Henan Province and the establishment of the slantwise moist isentropic surfaces in the lower troposphere in northern Henan Province. On 20 July, a low-latitude typhoon strengthened the southerly wind and enhanced the up-sliding movement along the slantwise moist isentropic surface, resulting in the rapid development of relative vorticity and heavy rainfall. The Tianji system can forecast this heavy rain process better than IFS and can provide scientific support for the meteorological departments to issue early warnings. Moist potential vorticity and slantwise vorticity development theory are powerful theoretical bases for heavy rain analysis and forecast, which are well related to precipitation intensity, position, and movement in reanalysis and model forecast data.