A record-breaking extreme Meiyu rainfall occurred along the Yangtze River valley (YRV) in 2020 since 1961, per-sisting from 11 June to 31 July with the largest amount and the highest intensity. From the aspect of water vapor, the causes of its formation are revealed in this study. The 2020 Meiyu rainfall amount is directly attributed to the greatly enhanced vertically integrated water vapor transport (IVT) convergence, which is in turn primarily determined by the mean circulation dynamic (MCD) contribution associated with anomalous East Asian summer monsoon (EASM) and the thermodynamic component (TH) contribution due to water vapor anomaly. The MCD contribution is mainly re-sponsible for the extreme Meiyu rainfall amount and abundant water vapor convergence in the YRV, whereas the TH contribution tends to shift Meiyu rain belt northward to the Yangtze–Huaihe River valley, extending the Meiyu rain-fall coverage area. Furthermore, the excessive moist static energy (MSE) associated with the largest water vapor an-omaly could substantially increase the atmospheric instability, favoring the extreme 2020 Meiyu rainfall intensity. In terms of the tremendous IVT to the YRV from both the South China Sea and Bay of Bengal during the 2020 Meiyu period, the low-level anticyclone anomalies over the western North Pacific (WNP) and Bay of Bengal provide appro-priate atmospheric circulation conditions, and they are generated by the super suppressed WNP convective activities as a Matsuno–Gill type response, which are further attributed to the combined warm SST anomalies in both the trop-ical western Indian Ocean (TWIO) and tropical Atlantic Ocean (TAO) eventually.