Abstract: Affected by the intensifying human activities and frequent occurrence of extreme climate in recent years, the problem of ancient landslide reactivation is increasingly prominent, threatening the local engineering facilities and urban security. Based on the Shangyaogou ancient landslide in the east margin of the Tibetan Plateau, the reactivation characteristics are analyzed, and the instability probability of the landslide under different rainfall conditions for return period is calculated. In addition, DAN^3D software is used to simulate the movement process and accumulation scope of the sliding mass, and assess the landslide hazard. The results show that: (1) A local part H1 has been reactivated in the front of the ancient landslide due to heavy rainfall and foot erosion, presenting the activity characteristics of multi-stage and multi-period. At present, the deformations on the slope are obvious, and have a potential for further instability. (2) H1 is in an unstable state under the condition of 20 years of continuous rainfall in 10 days with the instability probability of 99.12%. The posterior part H2 is highly likely to follow H1 and move downslope under the condition of 100 years of continuous rainfall in 10 days with the instability probability of 96.36%. (3) The farthest movement distance is about 350 m when only the reactivated part H1 moves, and the forefront of the landslide deposits will not reach the residential areas. (4) When the posterior part H2 moves following the reactivated part H1, the farthest movement distance is about 550 m. For the residential areas, the fan area from the head of the drainage channel to the national road G213 with a radius of about 150 m is considered to have a high hazard risk of landslide-debris flow.