Abstract: Frequent drought-to-flood abrupt alternation (DFA) events pose a severe threat to regional water security, yet systematic research on their spatiotemporal evolution and driving mechanisms in Southwest China remains scarce. Taking Southwest China (Sichuan, Chongqing, Yunnan, Guizhou, and Guangxi) as the study area, this paper employs the Standardized Drought-to-Flood Abrupt Alternation Index (SDWAI), combined with trend and change-point detection, spatial statistics, and a random forest model, to systematically analyze the spatiotemporal evolution and dominant driving factors of DFAA events during 1980-2020. The results show that: (1) the intensity and frequency of DFA events in Southwest China have significantly increased over the past four decades (Slope = 0.0209/year), with an overall abrupt shift occurring in 1998, when the dominant pattern changed from flood-to-drought (PTD) to drought-to-flood (DTP). The timing of abrupt changes varies among provinces, reflecting both regional consistency and heterogeneity; (2) significant spatial differentiation is observed: central Sichuan, western Chongqing, and central-eastern Yunnan are hotspots for DTP events, while northern Sichuan, southeastern Chongqing, and western Yunnan are more prone to PTD events. Humid zones tend to experience PTD, whereas semi-arid zones are more susceptible to DTP; (3) regarding driving mechanisms, precipitation (PRE) is identified as the most influential factor for DFA events (SDWAI) across the entire Southwest and its surrounding provinces, consistently exerting a significant positive effect. In addition, atmospheric pressure (PRS), relative humidity (RHU), wind speed (WIN), sunshine duration (SSD), minimum temperature (LOWTEM), and maximum temperature (HIGHTEM) exhibit widespread negative contributions, jointly forming a regulatory network of multi-factor synergistic effects. These findings provide important insights into the spatiotemporal heterogeneity and climatic drivers of DFAA in Southwest China, offering a scientific basis for regional water resource management and disaster prevention.