In mid-late July 2021, an extreme heavy rainfall process with the characteristics of long duration, large accumulated rainfall amount, and concentrated precipitation area occurred in Henan Province, causing serious casualties. Based on automatic station rainfall data and ERA5 reanalysis data, the important functions and influence mechanisms of multiscale system, jet and topography on the transport and convergence of water vapor and the formation of precipitation are discussed in this paper. It is found that the severe torrential rain occurred in the favorable circulation conditions with long-distance typhoon (In-fa). A large amount of water vapor coming from the western Pacific penetrated the east border of Henan Province from lower layer, while the water vapor from the South China Sea entered Henan Province from the south side mainly in the middle-low layers of troposphere, resulting in the extreme heavy rainfall under the joint influence of cyclonic vortex, shear line and convergence line. The centers of water vapor convergence and ascending motion in lower layers were caused by the coupling impact of low-level jet and boundary layer jet. In addition, the terrain here played the roles of dinamically blooking and lifting, and thermodynamically lifting at the same time. Besides, combined with the jet, topography made the severe precipitation present as a band shape in front of the mountain, and located in the central part on the 20th July and in the northern part of Henan Province on 21 July 2021.