Abstract: The "7·20" extreme rainstorm in Zhengzhou City in 2021 caused serious disasters such as urban waterlogging and watershed floods, resulting in heavy casualties and property losses. In order to comprehensively model and analyze the flood disaster process caused by the "7·20" heavy rainstorm in Zhengzhou City, a hydrological model of the Jialu River Basin in Zhengzhou City based on HEC-HMS model is constructed on the basis of field investigation the Jialu River Basin, by selecting the Jialu River Basin (a typical watershed in Zhengzhou) as the study area. The flood process caused by the "7·20" extreme rainstorm is simulated and the distribution of the rainstorm and the runoff contribution of the major tributaries in the basin are analyzed. In addition, the accumulated and stagnant water volume in the urban area is calculated by using the water balance theory. The results show that this extreme rainstorm has the characteristics of large rainfall magnitude, long duration and wide distribution range. From July 19 to 22 in 2021, the cumulative rainfall in the basin were 998 million m³, the average areal rainfall reached 495.24 mm, and the centroid of rainstorm was located in Erqi District. The Suoxu River benefits from the joint operation of the reservoirs with a smaller real runoff (10 million m³), the Jinshui River and Chaohe River have the largest runoff (21 million m³ and 24 million m³). the accumulated stagnant water in the urban area was some 140 million m³ with the average stagnant water depth of 0.37m. The research result can provide a useful reference for the formulation of urban flood disaster response measures.