On 8 February 2025, a catastrophic high-elevation rockslide occurred in Junlian County, Sichuan Province, Southwest China. The landslide mass transformed into a high-mobility debris avalanche, traveled approximately 1,200 m downslope with a vertical drop of 400 m. To investigate the dynamic evolution of this event, we integrated seismic signal analysis from nearby stations, detailed field observations, and Massflow numerical simulation. Integrated field and simulation analyses reveal four characteristic phases in the 50-s landslide dynamics: (1) initial shear detachment at the crown area (0-5 s); (2) downslope propagation with substantial material entrainment (5-20 s); (3) topographic deflection and partial deposition (20 s) and (4) progressive deceleration and final emplacement (20-50 s). This phased evolution highlights the transition from structural failure to mass flow behavior, with entrainment processes dominating the mobility enhancement. The calibrated simulation suggests a total runout duration of approximately 50 s, with a peak velocity of 34 m/s. Through comparative analysis of scenarios with/without erosion effects, we demonstrate that the simulation incorporating the entrainment effect yields deposition patterns showing remarkable consistency with both field observations and seismic signal distribution characteristics. This study highlights the value of integrating multi-source observations with numerical modeling to improve understanding and runout prediction of long-runout debris avalanche.