Abstract: Debris-flow disasters are a major geohazard impacting the safe operation of long-distance oil and gas pipelines in mountainous regions. In recent years, frequent debris flows and landslides along pipelines have heightened concerns about pipeline safety and disaster mitigation. To assess the risk posed by a single debris flow, 15 quantitative factors for evaluating debris-flow susceptibility, along with pipeline vulnerability and environmental impact assessment indicators categorized into high, medium, and low levels, were established based on the likelihood of occurrence, potential pipeline damage, and environmental consequences from post-disaster leakage. A corresponding risk assessment system was then developed. Taking the debris flow at the entrance of the Makouhe Tunnel on the Sichuan-to-East Gas Transmission Pipeline as a case study, the risk level was evaluated to support informed decision-making for pipeline safety and disaster prevention.