Abstract: In mountainous areas under extreme rainstorm scenarios, the concurrent occurrence of flash floods and debris flows triggers disaster chain effects. Traditional single hydrological-hydrodynamic models often underestimate inundation risks due to their neglect of terrain alterations. Taking Changping District, Beijing, as the study area, this paper constructs a coupled simulation model for the flash flood-debris flow disaster chain based on TRIGRS, MASSFLOW, and VMR-TRITON. First, the applicability of the VMR-TRITON model in Changping District was validated using the flash flood event of August 9, 2024.Second, debris-flow-induced terrain deformation is incorporated into the hydrodynamic model by dynamically updating terrain elevation over time. Finally, the model was verified using Hantai Village, a severely affected area during the "23·7" catastrophic rainstorm. The results indicate that the average relative simulation error of the VMR-TRITON model in Changping District is approximately 18.98%, demonstrating good applicability. Compared with simulation results that ignore channel deposition, the coupled flash flood-debris flow model improved the relative error in the simulated inundation area of Hantai Village from -52.66% to -22.98%, significantly enhancing accuracy. This study confirms the necessity of considering debris flow silting effects in simulations and provides a scientific basis for the refined defense against flash flood disasters.