Abstract: Objectives: On 7 August 2025, an extreme rainfall event struck Yuzhong County, Gansu Province, China, triggering multiple rapid-onset cascading hazards and resulting in severe casualties and property losses. To reveal the cascading characteristics and dynamic evolution mechanisms of this event, the Jiuzhuanggou catchment was selected as the study area. Methods: An integrated approach combining remote sensing interpretation, field investigations, and RAMMS numerical simulations was employed to systematically analyze the spatial structure and dynamic evolution of rainfall-induced cascading hazards under extreme precipitation conditions. Results: The cascading hazards triggered by the "8·7" extreme rainfall in Yuzhong County exhibited pronounced clustering, broad spatial extent, and cascading behavior, forming a typical cascading hazard process of “flash flood–landslide–natural dam formation–dam breach–debris flow.” Shallow sliding masses, loose slope deposits, and channel alluvial materials continuously supplied sediment throughout different stages, leading to a long cascading process, sustained sediment supply from multiple sources, and progressive amplification. Numerical simulations showed that the maximum flow velocity reached 16.64 m/s and the maximum deposition depth was 5.62 m. With an overall simulation accuracy of 80.2%, the model effectively reproduced the dynamic evolution of the cascading hazard. The results further indicate that blockage–breach processes in narrow channel sections were the key mechanisms controlling the formation and amplification of cascading hazard. Conclusions: This study improves understanding of how cascading hazards develop under extreme rainfall in semi-arid mountainous areas and offers insights for hazard identification, monitoring and early warning, and disaster prevention and mitigation in similar mountain catchments.