Abstract: In July 2021, a rare rainstorm occurred in Kaifeng city, causing the Huji River to run beyond the warning level for a long time. Based on the statistical analysis of rain and water situation in Kaifeng city, the new extreme value of historical hydrology characteristics in this area is summarized, and the conclusion is drawn that the frequency of “7·20” rainstorm in Kaifeng city is more than one thousand years.Based on the comparative analysis of flood formation factors in the historical and analysis years of the Huiji River, the reason why the “7·20” rainstorm in 2021 did not form the 1977 flood is explained.