Abstract: This study investigates the occurrence patterns and spatiotemporal evolution characteristics of drought–flood abrupt alternation (DFAA) events in the Taihu Lake Basin, providing a scientific basis for regional water-resource management, drought and flood mitigation, and ecological protection. As a core area of the Yangtze River Delta and one of China’s most economically developed regions, the basin is densely populated and highly sensitive to water availability and water security. DFAA events frequently trigger severe compound disasters, underscoring the need for detailed analysis. Using precipitation data (1951–2020) from hydrological stations in the basin—comprising daily rainfall for the basin’s areal rainfall, each sub-basin (Huxi, Wuchengxiyu, Yangchengdianmao, Taihu Lake Area, Hangjiahu, Zhexi, Pudong–Puxi), and 96 representative rainfall stations—this study identifies historical DFAA events through the Long-term Drought–Flood Abrupt Alternation Index (LDFAI) and the Short-term Drought–Flood Abrupt Alternation Index (SDFAI). Linear regression and wavelet analysis were employed to examine the spatiotemporal distribution patterns and evolutionary trends of DFAA events in the region. The main research findings are summarized as follows: (1) DFAA events in the Taihu Lake Basin can be classified into eight types: "spring drought to summer flood," "summer drought to autumn flood," "autumn drought to winter flood," "winter drought to spring flood," "continuous spring–summer drought to autumn flood," "continuous summer–autumn drought to winter flood," "continuous autumn–winter drought to spring flood," and "continuous winter–spring drought to summer flood." Among these, "spring drought to summer flood" has the highest probability at 37.8%, whereas "summer drought to autumn flood" is the least likely at 22.2%. Additionally, the likelihood of continuous drought spanning two seasons is generally higher than that of drought-to-flood transitions; for example, the probability of summer drought following spring drought is 48.2%, exceeding the probability of a summer flood transition (37.8%). (2) On a long-term scale, flood-to-drought events dominate the DFAA occurrences in the Taihu Lake Basin, with a clear upward trend in frequency. From 1951 to 2020, 13 flood-to-drought events and 13 drought-to-flood events were recorded, with the average intensity of flood-to-drought events (1.98) slightly higher than that of drought-to-flood events (1.92). Periodic characteristics show stage-specific differences: no significant periodicity was detected before 1990, whereas a roughly 10-year cycle appeared between 1990 and 2020, evolving through four distinct stages. On a short-term scale, flood-to-drought events also occur frequently, predominantly in autumn and winter. Short-term DFAA events are mainly concentrated between January–May and September–November. Within this period, drought-to-flood events peak in February–March (22.86%) and May–June (18.57%), while flood-to-drought events reach their highest frequency in September–October (24.29%), which also corresponds to multiple severe flood-to-drought occurrences. (3) Regarding spatial evolution, the distribution of long-term DFAA events exhibits a “north-high, south-low” pattern. Drought-to-flood events occur more frequently in the northwest, including Huxi District and Wuchengxiyu District (26–27%), whereas flood-to-drought events dominate the southeast, such as Hangjiahu District and Pudong–Puxi District (26%). Short-term DFAA events show more significant regional differences. In February–March, the pattern is “south-high, north-low,” with the highest frequency in Hangjiahu District (36%). In May–June, events are concentrated in the north, with Yangchengdianmao District (34%) as the core, mainly consisting of drought-to-flood events. In September–October, the entire basin experiences a high-frequency period, with an average frequency of 30%, and Pudong–Puxi District reaches the highest frequency (40%), mainly dominated by flood-to-drought events (average 26%). In conclusion, this study clarifies the spatiotemporal evolution characteristics of DFAA events in the Taihu Lake Basin, filling a gap in systematic research on such events and providing technical support for regional management of extreme hydrological events and the formulation of emergency response plans.