In the eastern Tibetan Plateau, numerous landslides triggered by rainfall occur annually during the rainy season, posing serious threats to lives and properties in mountainous towns and cities. Quantitative evaluation of landslide risk is the focus of geological disaster prevention and control. Using the Deda ancient landslide on the right bank of the Baqu River in the upper Jinsha River as an example, on-site detailed investigations, drone aerial surveys, and numerical simulations were conducted to identify the deformation of the Deda ancient landslide, analyze landslide destabilization probabilities under different rainfall recurrence periods, determine the scope of landslide threats via numerical simulation, assess the vulnerability of affected structures within the impact area, and quantify risk values for buildings and people across various rainfall recurrence periods. The risk values of buildings and people under different rainfall return periods are quantitatively calculated. The results indicate that 28 buildings in the study area are at risk from landslides, with over 69% of structures having a susceptibility greater than 0.8 and a population susceptibility index of 0.596. The probability of destabilization gradually increases under natural, 20-year, 50-year, and 100-year rainfall conditions, with annual property losses estimated at 68,100 yuan, 532,000 yuan, 791,000 yuan, and 927,000 yuan, respectively, and estimated casualties of 3, 17, 25, and 30 people, respectively. The results of the study quantify the loss after the occurrence of landslides, which is an important reference for the prevention and control of this type of landslides, and can provide disaster prevention and mitigation suggestions for relevant departments.