Abstract: Using daily precipitation data from 86 national meteorological observation stations in Guangdong from 1961 to 2024, together with NCEP/NCAR reanalysis data, NOAA ERSSTv5 sea surface temperature (SST) data, and monthly outgoing longwave radiation (OLR) data, we calculated the short-term drought–flood abrupt alternation index (SDFAI) for Guangdong to analyze the characteristics of drought–flood abrupt alternation over the past 64 years. Composite and correlation analyses were then applied to examine concurrent atmospheric circulation anomalies and preceding SST anomalies associated with SDFAI anomalies in Guangdong. The analysis focused on March–April, the period during which the probability and intensity of drought–flood abrupt alternation were highest over the past 64 years. The results show that, over the past 64 years, the highest probability and intensity of short-term drought–flood abrupt alternation between two consecutive months in Guangdong occurred in March–April, whereas the lowest occurred in January–February. The SDFAI for March–April exhibited significant interannual and interdecadal variability, with the highest value in 1973 (3.43) and the lowest in 1968 (−2.49). In 26.6% of the years, drought–flood abrupt alternation occurred during March–April. The index showed a significant decreasing trend of 0.15 (10 a)⁻¹, with relatively high values during 1961–1990 and relatively low values during 1991–2024. After detrending the SDFAI, high-index and low-index years for drought–flood abrupt alternation in March–April were selected. Composite analyses of atmospheric circulation for these two groups revealed significant differences. During the drought period in March, high-index years (compared with low-index years) exhibited strengthened convergence in the upper troposphere, an intensified East Asian trough in the mid-troposphere, stronger cold air activity, and anomalous northerly flows over South China in the lower troposphere. At the surface, both the Siberian High and the Aleutian Low were significantly stronger. Subsidence was pronounced over Guangdong, and low-level divergence dominated South China. Moisture transport from the Bay of Bengal and the South China Sea to South China weakened, resulting in significantly reduced precipitation in Guangdong in March. During the flood period in April, the atmospheric circulation patterns were nearly opposite to those in March, producing anomalously high precipitation. In the preceding winter, higher SSTs occurred in the tropical western Pacific, while lower SSTs were observed in the South China Sea, on the western side of the Maritime Continent, and in the northern Indian Ocean. Lower SSTs in the equatorial central and eastern Pacific during the preceding autumn and winter (indicating a decaying or persistent La Niña event) strengthened the Walker Circulation. An anomalous cyclonic circulation formed near the Philippines, placing South China under the influence of anomalous northerly flows and within a subsidence region, which contributed to reduced precipitation in March. In April, the rise in SSTs in the tropical western Indian Ocean favored moisture transport from the Arabian Sea and the Bay of Bengal to South China. The increase in SSTs in the tropical western Pacific helped maintain the anomalous anticyclonic circulation east of the Philippines, thereby strengthening the western Pacific subtropical high. South China then came under ascending motion, with convergence of cold and warm airflows in the lower troposphere and enhanced convection. This led to anomalously high precipitation in Guangdong in April and the occurrence of drought–flood abrupt alternation.