Abstract: During flash flood formation, abundant loose materials produced by rainfall-induced landslides and slope erosion enter the river channel with runoff, significantly altering flood routing characteristics and transforming into flash flood and debris flow disasters. On June 26, 2020, short-duration intense rainfall combined with steep terrain triggered local landslides and catastrophic flash flood-debris flow events in the Caogu River Basin, Mianning County. To investigate the impact of landslide-induced sediment supply on the evolution of flash flood-debris flows, this study adopted a numerical simulation approach to analyze the distribution of potential landslide sediment sources in the Caogu River Basin under short-duration heavy rainfall. The propagation process of flash flood-debris flows was simulated using the spatial boundary of landslide sediment sources, and the effect of landslide-derived sediment on disaster formation was further examined. Results show that the short-duration intense rainfall triggered numerous landslides, which were mostly distributed along both sides of gullies. The northern gully provided a particular large volume of loose sediment. After discharging from the gully, flow velocity dropped abruptly due to a sudden decrease in channel gradient, resulting in massive sedimentation that ultimately caused river diversion and inundation of downstream villages. The findings reveal the key controlling effect of landslide sediment supply on the formation of flash flood-debris flow disasters, and provide a scientific basis for the prevention and mitigation of such hazards in mountainous areas of southwestern China.