Abstract: Heixiluo gully, located in Suxiong Town, Ganluo County of Sichuan Province, experienced a catastrophic debris flow disaster at 8:00 a.m. on August 31, 2020, causing significant losses to local residents, as well as to the Chengdu–Kunming railway bridges and infrastructure. To Study the activity and dynamic characteristics of the debris flow in Heixiluo gully, a comprehensive analysis was conducted using field investigations, on-site observations, and high-precision DEM data from the study area. The formation conditions and activity characteristics of the “8•31” debris flow were studied, and the Massflow software was utilized to simulate and verify the debris flow, inversely simulating the dynamic evolution process of the debris flow in Heixiluo Gully, and quantitatively evaluating the dynamic characteristics of the“8•31”debris flow. The study indicates that the “8•31” debris flow mainly underwent a“snowballing” cycle of processes, including “rainfall runoff convergence-, incision and erosion on the sides, sedimentation and selection on the first-level platform, erosion on the rear side of the platform, further sedimentation and selection on the second-level platform, further incision and erosion on the rear side of the platform, bank slope collapse, blockage and collapse, river blockage, formation of barrier lakes, and dam breach discharge.” Based on the Massflow analysis of the dynamic process of the debris flow, the simulated peak discharge, flow velocity, flow depth, erosion, and sedimentation depths in each gully segment match the measured data, confirming the reliability of this method. Through this method, the dynamic characteristics of debris flow can be more intuitively analyzed, providing a theoretical basis for subsequent disaster prevention and mitigation works.