Abstract: The Pearl River Basin is highly prone to rainstorm-induced flash flood disasters, characterized by pronounced spatial heterogeneity in both disaster-forming processes and hazard manifestation patterns. Based on integrated analyses of hazardprone geomorphic settings and typical flash flood events, this study examines disaster chain evolution, hazard source inventorying, monitoring and sensing, early warning modeling, and emergency response across five dimensions. The results show that the alpine gorge region of the upper reaches is characterized by a complex flash flood-debris flood chain. The YunnanGuizhou Plateau slope region features two coexisting patterns—karst waterlogging and landslide-debris flow transformation. The mid‑low mountainous region is marked by clustered flash flood-geological disasters, and the Hainan dome region is dominated by typhoon-driven flash flood inundation. Current defense systems in the region face several critical bottlenecks, including unclear thresholds for disaster chain transformation, incomplete hazard inventories, insufficient monitoring coverage, poor adaptability of early warning indices, and a lack of region-specific emergency plans. To address these challenges, a refined defense system with zoning and classification is proposed. For the alpine gorge region of the upper reaches, a trinity system integrating material source inventory, process sensing, and progressive early warning is recommended. In the Yunnan-Guizhou Plateau slope region, technical advancements are needed in subsurface river detection and early warning of landslide-debris flow transformation. For the mid‑low mountainous region, a dynamic clustered-risk field and a multi-objective optimization model should be developed. In the Hainan dome region, probabilistic early warning driven by typhoon ensemble forecasts and targeted push technology should be implemented.