Abstract: Extreme rainstorms occurred in some regions of Henan Province from 19 July to 22 July 2021 (referred to as “21·7” Henan rainstorm), causing severe urban waterlogging and casualties. Precipitation observation data and reanalysis data were used to analyze the large-scale circulation situations of this heavy rainfall. The moisture sources, transport paths, and quantitative contribution of each source area were also examined using the Lagrangian trajectory tracking model (FLEXPART) and the areal source-receptor attribution method. The results showed that the 500-hPa circulation in East Asia was extremely stable during the rainstorm and several days before the rainstorm. Connected with the northern high pressure ridge, the western Pacific subtropical high pressure (referred to as the sub-high) was unusually northward, with less movement. Meanwhile, the Eurasian high trough was westward, and the circulation in the middle and high latitudes of East Asia was significantly flat. During the rainstorm, tropical cyclones “In-Fa” and “Cempaka” continued to have synergistic effects with sub-high, establishing obvious water vapor transport channels and providing sufficient moisture. The rainstorm areas in Henan Province retained considerable near-surface wet regions and high precipitable water. Tracking forward for several days, the target particles in the rainstorm area mainly originated from the Northwest Pacific Ocean, the South China Sea, and other places and were located at relatively low atmospheric levels. Additionally, a few particles, which could be traced back to the vicinity of the Sea of Japan and central Eurasia, were at relatively high altitudes. Quantitative contribution analyses indicated that the moisture primarily originated from the continent of eastern–central China south of Henan (D) and the northwestern Pacific Ocean (F), with the contribution of the former (52.59%) twice more than that of the latter (25.51%). In addition, the moisture from the Henan rainstorm area (T, 3.68%), Indo–China Peninsula–South China Sea (E, 3.32%), and the Asian continent north of the storm area (B, 2.28%) also contributed. The water vapor uptake of the target particles was the largest in region D, which was slightly higher than that in region F, but the rate of moisture loss along the former was clearly lower than that of the latter, resulting in significantly higher moisture contribution in region D than in region F. Although the moisture intake in region B was somewhat higher than that in region E, the sum of the moisture loss along the way and the unreleased part was higher than the latter, leading to a greater moisture contribution in region E. Moreover, region T had a non-negligible precipitation recycling rate. When the number of forward tracking days was extended, the trajectories of target particles, water vapor uptake, and contribution rate of each source had relatively minimal changes, but the total contribution of moisture from all source areas significantly increased. Thus, the number of days for the tracking of moisture sources for heavy precipitation events, such as this extreme rainfall in Henan, can be extended.