Based on daily precipitation data collected at 567 stations in China from 1961 to 2015, an index of abrupt drought-flood alternation in Southwest China in summer was defined. The results show that the alternation events from drought to flood are more than those from flood to drought in the summers during 1961-1970, the alternations from flood to drought are more during 1971-1980, and the alternations from drought to flood are equal to those from flood to drought during 1981-2000. Since the early 21st century, abrupt alternations from flood to drought tend to increase. Typical abrupt drought-flood alternation events in summer were selected and the characteristics of the anomalous atmospheric circulation and water vapor transportation were analyzed. It was found that the western Pacific subtropical high extended westward and became stronger during the drought period before the conversion from drought to flood. The westerlies at the middle and high latitudes also became stronger, preventing cold air from moving southward. In the vertical, downdraft prevailed and the moisture transport from the Bay of Bengal and South China Sea was weaker than before. Southwest China was located at the moisture divergence area. In addition, there was less rainfall during drought periods. During the flood period after the conversion from drought to flood, meridional circulation at the middle and high latitudes tended to be stronger. The trough to the east of the Ural Mountains deepened and the eastern Asia ridge strengthened. The westerlies at the middle and high latitudes became weaker and updrafts prevailed in the vertical. Also, large amounts of moisture were transported from the Bay of Bengal and South China Sea to Southwest China, which explains why rainfall became abundant there. The opposite was true in the years of conversion from flood to drought.