Under the stable circulation of two troughs and one ridge in middle and high latitudes of Asia, the western North Pacific subtropical high and Typhoon In-fa and Cempaka lead the water vapor to converge in Zhengzhou, resulting in a rare extreme rainstorm with precipitation extremum more than 800 mm there from 19 to 21 July 2021 (hereinafter referred to as “7·20” extreme rainstorm in Zhengzhou), causing heavy casualties and property losses. The HYSPLIT model is used to track the water vapor source and transport characteristics of this extreme rainstorm process. The results show that the water vapor of “7·20” extreme rainstorm in Zhengzhou mainly comes from the west of the North Pacific and the north of the South China Sea, which are affected by the joint action of the western North Pacific subtropical high and Typhoon In-fa and the synergistic effect of Typhoon Cempaka and In-fa, respectively. The water vapor is mainly transported below 850 hPa, while the water vapor from the north of the South China Sea rises gradually in the process of northward transport and is close to 700 hPa near Zhengzhou. Based on the quantitative calculation of the water vapor trajectory by HYSPLIT backward tracking, it is found that the water vapor contributions from the west of the North Pacific and the north of the South China Sea account for about 73% and 27%, respectively.