Abstract: Using the sudden turn of drought and flood (STDF) in the middle and lower reaches of the Yangtze River during 2011, as an example, the relation between the STDF and circulation and sea surface temperature (SST) is studied. The reasons for the STDF are the following. (1) The atmospheric circulation quickly adjusted from strong winter monsoon to the station, called two troughs and one ridge. (2) The location and strength of the western Pacific subtropical high also changed quickly. The convergence of water vapor transport was obviously weaker in the middle and lower reaches of the Yangtze River from January to May. In June, the water vapor transport and budget changed and there was an obviously stronger center of water vapor convergence in the middle and lower reaches of the Yangtze River. (3) Because of the cyclonic anomalous circulation over the Tibetan Plateau, the convection is active and frequently moves to the middle and lower reaches of the Yangtze River. This convection strengthened the Mei-yu front and triggered rainfall five times. (4) The persistent La Niña event and its variations created favorable background conditions by influencing the Walker circulation, the western Pacific subtropical high, and so on.