<p>In high-mountain and steep-gorge regions, traditional hydrological methods often underestimate the peak discharge of dam-break debris flows because they simplify the chain-like transformation from landslide blockage to rapid breaching. This study focuses on the Yizhong River in Deqin, Yunnan, a typical landslide-blockage-dam-break disaster chain developed in a confined V-shaped gully. By integrating a gradual-breaching calculation model with Massflow dynamic simulations, the full process was quantified under extreme rainfall scenarios with 1% and 0.5% exceedance probabilities. The results show that topographic confinement produces a pronounced funnel effect, restricts lateral spreading, and promotes rapid conversion from stored potential energy to downstream kinetic energy during breaching. Under the 0.5% rainfall scenario, the simulated instantaneous peak discharge at the breaching point reaches 927.2&#xa0;m³/s, which is 15.3% higher than the theoretical value of 804.2 m³/s. Accordingly, a dynamic amplification factor (<i>α</i> = 1.15) is proposed to quantify the underestimation caused by conventional calculations. The results suggest that mitigation design for high-energy dam-break debris flows should shift from static interception to dynamic energy dissipation, and they provide a quantitative reference for similar steep-gorge basins in southeastern Tibet and other high-relief mountainous regions.</p>