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The record‐long 2020 Meiyu season since 1961 caused severe floods over the Yangtze and Huaihe River valleys (YHRV). Why the Meiyu duration doubled in 2020 remains a puzzle. We show that the long‐lasting Meiyu can be divided into three stages: advanced‐onset, strong‐persisting, and delayed‐withdrawal. The advanced‐onset was associated with an extremely negative‐phase East Atlantic/West Russia teleconnection. The strong‐persisting was attributed to a positive‐phase Pacific–Japan (PJ) pattern sustained by La Niña's rapid development. The delayed withdrawal was related to the combined effect of a positive PJ pattern and a mid‐troposphere "two ridge–one trough" pattern over Asia. Two subseasonal forecasting systems predicted the positive rainfall anomalies over the YHRV in the first two stages, but not the third stage, which may be associated with poor prediction of the two ridge–one trough pattern. These results highlight the role of subseasonal evolution in extreme climate events. Plain Language Summary: During the northward advance of the East Asian summer monsoon, the East Asian subtropical region is normally characterized by a quasi‐stationary rainband that persists from mid‐June to mid‐July, which is known as Meiyu in China and Baiu in Japan. Both the duration and the accumulated rainfall of the 2020 Meiyu set the highest record since 1961, which caused a severe flood and an enormous economic loss in the Yangtze and Huaihe River valleys (YHRV), central‐eastern China. We investigated the mechanism and predictions of this long‐lasting flood. The record‐long 2020 Meiyu can be divided into three stages: advanced‐onset, strong‐persisting, and delayed withdrawal. The advanced‐onset stage was mainly induced by the extratropical climate anomalies associated with the East Atlantic/West Russia teleconnection. In contrast, the strong‐persisting stage was due to the enhanced tropical forcing via triggering the Pacific–Japan pattern. The delayed withdrawal stage arises from the combined effect of tropical forcing and mid‐latitude teleconnection. The current advanced forecasting systems can predict the positive rainfall anomalies over the YHRV in the first two stages, but not the third stage. These findings advance our understanding of the extreme 2020 flood and help improve future extreme monsoon prediction. Key Points: The long‐lasting 2020 Meiyu season was characterized by three stages: advanced‐onset, strong‐persisting, and delayed withdrawalThe mid‐latitude teleconnection played a crucial role in the 2020 Meiyu onset and withdrawal, while tropical forcing's role is enhanced in the last two stagesAdvanced forecasting systems cannot predict the delayed withdrawal due to the failure in predicting the mid‐latitude teleconnection [ABSTRACT FROM AUTHOR]