The extreme Meiyu in 2020 was featured with earlier onset, later retreat, longer duration of rainy season, wider rainfall range, abundant accumulated rainfall, and more severe rainfall processes than normal. Through the analysis of simultaneous atmospheric circulation systems, we have found that the average locations of the key East Asian monsoon circulation systems were relatively stable, with significant characteristic of quasi-biweekly oscillation (QBWO) in the Meiyu season of 2020. The onset and retreat of Meiyu, the northward shift and stagnation of the rainfall belt, and the occurrence and persistence of severe rainstorm processes had a good relationship with the QBWO. During the Meiyu season, the western Pacific subtropical high experienced six periodic oscillations of northward migration and southward retreat. Meanwhile, the upper- and low-level monsoon circulation systems strengthened for five times corresponding with the QBWO in the Meiyu season. Especially, the strengthening of the southwesterly low-level jet, add the repeated establishment of the large-value centers of southerly with relatively consistent latitude made the water vapor transport from the tropics strengthened again and again, and the water vapor convergence and upward movement developed repeatedly, leading to the long-term persistence of Meiyu in the Yangtze-Huaihe River Valley (YHRV) and frequent occurrences of rainstorms. On the other hand, two blocking highs maintained in the mid- and high latitudes, which was characterized by the spatial pattern of “two ridges and one trough” over Eurasia, and the low trough along the East Asian coast was also active. The cold air flew continuously southward through the northwestern and/or northeastern paths, frequently merging with the repeatedly strengthened warm moisture from the low latitude over the YHRV. So, this is another important factor causing the extreme Meiyu in 2020. Moreover, by comparing the spatio-temporal distribution of Meiyu and the corresponding flood disaster in 2020 with other super Meiyu years since 1951, we have also found that the Meiyu in 2020 was weaker than that of 1954, but stronger than that of 1998 and 1991. Thanks to the improvement of both the accuracy of climate prediction and the ability of disaster prevention and mitigation, the flood and disaster losses caused by the 2020 extreme Meiyu in the YHRV were significantly less than before.