[Objective] The development characteristics of the debris flow in Nangang gully， Fangshan District， Beijing City were analyzed to reveal the differences in material source composition and disaster-causing mechanisms of rainstorm-induced debris flow in the southwest mountainous areas of Beijing City， in order to provide theoretical support for debris flow prevention and control in the local area fand similar regions. [Methods] Taking the debris flow disaster that occurred in Nangang gully after the ‘23·7’ rainstorm in Beijing as the research object， topographic data were acquired using unmanned aerial vehicle （UAV）， and terrain， material source， and water source conditions were analyzed in combination with field investigations. The FLO-2D model was used to simulate the dynamic process of the debris flow， with a focus on quantifying movement parameters under the dendritic gully-high and steep slope topography. The results were then validated against field investigation data. [Results] Nangang gully featured large topographic relief and deeply incised valleys， with abundant and diverse material sources （including landslide deposits， slope erosion deposits， and channel deposits）. Atmospheric precipitation served as the main water source， and rainstorms were the primary triggering factor for debris flow. The FLO-2D simulations showed that the maximum sliding velocity of the debris flow was 6.75 m/s， and the maximum accumulation depth was 14.6 m. The error rate between the simulation and field investigation results was 3.995%， which was within a reasonable range. A comprehensive analysis confirmed that the susceptibility index of debris flow in Nangang gully was 2—3， classifying it as a debris flow-prone gully. [Conclusion] The Nangang gully debris flow is a typical rainstorm-induced gully-type dilute debris flow， with its formation following the process of ‘rainfall infiltration—runoff concentration—channel bed scouring—material convergence’， and the gully is classified as a debris flow-prone area. The FLO-2D simulation has been verified to be reliable.