In response to the “July 19” mudslide disaster in Haizigou, Rixiang Township, Xiaojin County, Sichuan Province, field investigations, LiDAR and UAV photogrammetry (with a 0.1 m resolution DEM generated), and numerical simulation (via the Massflow platform coupled with the Voellmy friction model) were integrated in this study to systematically reveal the chain reaction disaster mechanism and the evolution of its hazard. The results indicate that the disaster was driven by the “dual rainfall synergy effect”: prolonged rainfall (260.3 mm over 30 days) saturated the source material, while short-term heavy rainfall (with over 23.4 mm recorded in 1 hour in the upstream area) triggered a chain reaction of landslide blockage and breach, leading to the formation of a typical progression path: “runoff convergence → tributary blockage and breach → upstream erosion and lateral erosion of the main channel → sediment accumulation in the plateau section → secondary erosion in the narrow section → culvert blockage and water level rise → channel mouth blockage and flood disaster”. The dynamic response was quantitatively revealed: the water flow velocity in the narrow section exceeds 6 m/s. When the rainfall return period changes from the 10-year to the 100-year scenario, the proportion of high-velocity regions (>4 m/s) rises from 12.48% to 21.83%, the deposition width expands from 182.8 m to 662.4 m, and the maximum sediment depth increases from 2.7 m to 6.1 m. According to the zoning method based on the dual indicators of H_max and Hv_max, the area of high-risk zones in the river channel under the 100-year return period event (where H_max≥ 2.5 m or Hv_max≥ 2.5 m/s) reaches 29 661 m², representing a 20.6-fold increase compared with that under the 10-year return period scenario.The applicability of the 3D real-scene model and the Voellmy model (μ=0.1, ζ=200 m/s²) in high-mountain canyon areas was validated in this study, and a quantitative basis was provided for disaster prevention planning.