Abstract: With the increasing frequency of extreme rainfall events, flash flood disasters have occurred many times in the mountainous areas of Beijing in recent years. From August 18 to 19, 2024, a localized area in Pinggu District experienced a short-term heavy rainfall, causing flash flood disasters in Xiangyang Village and Shuiyu Village of Jinhaihu Town. To further analyze the causes and characteristics of flash flood disasters, relevant disaster basic data were obtained through on-site key cross-section measurements, flood mark measurements, and field investigations. The characteristics of rainfall and flood were simulated and analyzed using models. The results show that the catchment area of basins where Xiangyang Village and Shuiyu Village are located is 7.46 km². The total rainfall of this event is 170 mm, reaching the magnitude of a 20-year return period. The peak flood discharge at the upper, middle and lower outlets of Xiangyang Village is 7.89 m⁵/s, 8.37 m⁵/s and 19.57 m⁵/s respectively, with a flood recurrence interval of less than 10 years. Continuous rainfall is the direct cause of this flash flood.Meanwhile, in the small watersheds of flood discharge channels in mountainous areas, the water gathering surface is small, the relative height difference is large and there are many obstacles, resulting in high potential energy and fast flow speed of flash flood. The encroachment of channels has led to serious local water backflow and overflow. Due to the occupation of the channel,the local backwater overflows seriously. According to the investigation results, countermeasures and suggestions are put forward in terms of resilient disaster reduction, village planning, refined management, regulation of flood passage space, etc.