Abstract: Objective Debris flows are a major geological hazard that threatens the safety of people's property. In late July 2023, 132 flash floods and debris flows were triggered in the Xishan area, causing significant impacts. However, current research on the genesis and mechanism of debris flows driven by extreme rainfall in typical northern regions remains relatively weak. Methods Taking the Nan’an Basin in Nanjiao Township, Fangshan District, Beijing as the research object, this paper focuses on analyzing the particle size distribution, non-uniformity coefficient, and curvature coefficient of loose channel materials based on field investigations, particle size analysis, and rainfall monitoring data, and investigates the formative conditions and disaster-causing mechanisms of debris flows. Results The results indicate that the Nan'an watershed covers an area of 12.23 km²and an average longitudinal slope of the main channel of 151‰. The development of tributary gullies and the broad-flat shape of the watershed cause concentrated and rapid confluence, easily forming high peak flows and providing strong topographic power for the initiation and transport of debris flows. The loose deposits within the channel, with a static reserve of 1.59 × 10⁵ m³, are mainly composed of gravelly soil, characterized by poor gradation, and weak inter-particle connectivity, providing the material basis for the occurrence of debris flow disasters. The "23·7" severe rainstorm had a cumulative rainfall of 949.4 mm, with a maximum hourly rainfall intensity of 65.2 mm/h. The short-duration extreme rainfall intensity combined with soil saturation reduced the shear strength of the source material, and the intensified scouring and undercutting capacity directly triggered the debris flow. The disaster mechanism of this event is manifested as a triple synergy of "topographic empowerment - material supply - hydrodynamic triggering", which jointly facilitated the occurrence of this dilute, gully-type, coarse-grained debris flow induced by rainstorms. Conclusions Debris flows in northern mountainous areas result from high‑intensity runoff induced by extreme rainfall that scours and entrains pre‑existing loose channel deposits, producing a coarse‑grained, dilute debris flow.