Abstract: From late August to early October 2021, the rainy day lasted for more than 40 days in the middle reaches of the Yellow River, and seven heavy precipitation processes occurred, the cumulative rainfall of Jing-Wei River、 Beiluo River、 Fenhe River and Sanmenxia to Huayuankou Interzone are 2 to 5 times in the same period in the history, both were the maximum rainfall since the observed data. Influenced by plenty of rainfall,the most serious autumn flood occurred in the Yellow River since 1949, in this period, three numbered floods occurred during nine days in the middle and lower reaches of the Yellow River, the maximum flood peak occurred at Tongguan hydrological station since 1979, Weihe River、 Qinhe River and Yi-Luo River appeared the largest flood process in the same period in September, and the same situation of Fenhe River and Beiluo River in October. In this paper, the flood data and NCEP/NCAR data are used to statistics the main precipitation characterstics and flood process of the Yellow River Basin in 2021, and analyze the precipitation weather situation of the autumn flood. The results show that during the autumn flood season of this year, the synoptic circulation of lower in the west and higher in the east has the mainly influence on the mid high latitude of Asian continent. The low trough from Balkash Lake to Baikal Lake is lower, and the ridge of pressure located in East Asia is stronger. Meanwhile the subtropical high over northwest Pacific is stronger and its position is farther northern, with broad low pressure areas over South Asia, which formed a favorable cold-warm drafts converge conditions under the stable weather situation in the middle Yellow River. The water vapor transport mainly depends on the combination of the northwest Pacific subtropical high and the depression over the bay of Bengal, and the middle Yellow River is extermely strong water vapor flux convergence.