This study analyzes the water vapor transportation features, water vapor sources, and key synoptic-scale systems of the “7.20” rainstorm in Henan in 2021. Double typhoons “In-fa” and “Cempaka” and the western Pacific subtropical high jointly provided sufficient water vapor conditions for the “7.20” rainstorm in Henan. However, the extreme rainstorm event, which has daily precipitation of 663.9 mm and 1-hour maximum precipitation of 201.9 mm on July 20, can hardly be explained only by the roles of typhoons and western Pacific subtropical high. Results of the water vapor flux analysis and trajectory analysis based on the LAGRANTO model show that a strong northward water vapor flux zone (above 850 hPa) was formed on the southern side of Henan on July 20, 2021. It then converged with the low-level water vapor flux zone facilitated by the typhoon and western Pacific subtropical high near Henan, thus providing the most abundant water vapor conditions for the rainstorm. This work emphasizes that the anticyclonic wave-breaking event that occurred at the tropopause to the west of Henan on July 20 triggered a strong meridional water vapor flux on the southern side of Henan and worked synergistically with typhoons, resulting in this extreme rainstorm.