Abstract: In July 2023, a rare heavy rainstorm occurred in the Beijing-Tianjin-Hebei region, which caused heavy flash floods and urban waterlogging disasters in Mentougou and other districts in Beijing. The Mentougou watershed has an area of only 37 km², and there is no available hydrological monitoring station, which brings challenges to flood analysis and simulation. By using the surveillance video of flood processes in key sections, as well as hydrological simulation based on digital watershed model through post-disaster on-site investigation and measurement, the maximum peak discharge of Mentougou watershed was estimated as 512.6 m³/s occurred on July the 31st. During the flood peak process, the discharge under the Northwest Cross River bridge with three hole box culvert near Zhong'ang Times Square, which controls 90% of the watershed area, was between 215.0 m³/s and 308.9 m³/s, which was far lower than this maximum peak discharge of the section. For the future recovery and reconstruction of the disaster, the design flood discharge of the bridges in Mentougou and Fangshan districts should be comprehensively reviewed, and the flood water passage in the section of the bridges should be enlarged so that the extreme rainstorm-flood could pass safely.