Debris flows pose serious risks to communities in mountainous areas, often resulting in large losses of human life and property. The impeding presence of urban buildings often affects the runout behavior and deposition of debris flows. But the impact of different building densities and sizes on debris flow dynamics has yet to be quantified to guide urban planning in debris flow risk zones. This study focused on a debris flow that occurred in Zhouqu County, Gansu Province, China on August 7, 2010, which was catastrophic and destroyed many buildings. The FLO-2D software was used to simulate this debris flow in two scenarios, i.e. the presence and the absence of buildings, to obtain debris-flow intensity parameters. The developed model was then used to further analyze the influence of large buildings and narrow channels within the urban environment. The simulation results show that considering the presence of buildings in the simulation is essential for accurate assessment of debris flow intensity and deposition distribution. The layout of buildings in the upstream urban area, such as large buildings or parallel buildings which form narrow channels, can affect the flow velocity and depth of debris flow heading towards downstream buildings. To mitigate damage to downstream buildings, the relative spacing (d/a) between upstream and downstream buildings should not exceed a value of two and should ideally be even lower. These findings provide valuable insights for improving the resistance of mountainous cities to urban debris flows.