Abstract: In recent years, extreme rainfall has occurred frequently in mountainous areas of Southwest China, and debris flows have posed a serious threat to masonry buildings. This study aims to establish a refined risk assessment method based on the characteristics of hazard-affected bodies. Combining field detailed survey, unmanned aerial vehicle (UAV) aerial survey and Massflow numerical simulation, the dynamic evolution process of debris flows under different rainfall frequencies (P = 1%, 2%, 5%, and 10%) is reproduced, and key parameters such as flow velocity and flow depth are obtained. The burial failure mode of debris flows on masonry buildings is analyzed, and a vulnerability model based on burial depth is constructed. Finally, refined risk assessment of debris flows in the study area is carried out. The results show that at the rainfall frequency of P = 1%, the maximum flow depth of debris flow reaches 4.11 m and the maximum flow velocity is 11.2 m/s. With the rainfall frequency decreasing from P = 5% to P = 1%, the number of high-vulnerability and extremely high-vulnerability buildings in the study area increases significantly to 21 and 18, respectively. At P = 1%, the total area of high-risk and extremely high-risk zones increases by 194.74% (from 1.9×10⁴ m² to 5.6×10⁴ m²), and the number of high-risk and extremely high-risk buildings increases by 40. The results can provide a scientific and theoretical basis for debris flow disaster prevention, and mitigation and emergency route planning.