In early October, 2021, northern China experienced an unprecedented autumn flood in history.From October 3 to 6, persistent extreme heavy rainstorm maintained stably in the regions centered along the middle-lower reaches of the Yellow River and Hai River (including the south of central Shaanxi-Shanxi-Beijing-Tianjin-Hebei-Liaoning and the north of Shandong).The flood caused huge economic losses and serious casualties.Based on the observed precipitation in China, the NCEP/NCAR and ERA5 reanalysis datasets, the extreme of this heavy rainfall case is diagnosed in this paper.Results show that the rainstorm process shows the characteristics of typical northern summer rainstorm and atmospheric circulation in terms of precipitation intensity, duration and meridional water vapor transport.The regional average 4-day accumulative rainfall intensity of the above five provinces and two cities is much higher than that of other rainstorms in autumn from 1961 to 2020, and ranks second even in summer.Its extreme intensity is closely related to the long-term persistency of heavy precipitation center in the above areas.The regional average precipitation of the above five provinces and two cities exceeds 15 mm for four consecutive days, which has never occurred in the history of autumn.In addition to the strong extremes of the process, the abnormally abundant precipitation in Shanxi and other places in September also superimposes the autumn flood in early October.The atmospheric circulation corresponding to this autumn flood presents a typical circulation pattern in the main rainy season in summer in northern China, which is manifested by a much stronger and more westward/northward western Pacific subtropical high, and the meridional water vapor transport in the west of the subtropical high is also extremely strong.At the same time, a strong cold air process invades northern China from October 4 to 6, and the cold dry and warm wet air flows converge in the above areas.The calculation of water vapor budget shows that the meridional water vapor transport intensity of this process is the highest in autumn history, and even exceeds that of most rainstorm processes in northern China in midsummer.Above results show that even in mid-autumn, the circulation pattern and water vapor transport conducive to extreme persistent rainstorm in northern China can also be triggered, resulting in severe autumn flood.