Abstract: To study the deformation characteristics and the triggering-mechanism of ancient landslide, the Outang landslide is selected as a research object and its temporal-spatial deformation characteristics, corresponding trigger factors and reactivated mechanism are examined based on detailed analyses of geological materials obtained by using boreholes, trench, adits and monitoring data sets as well as the simulation numerical model. Electron spin resonance tests and in-site investigations show that the Outang landslide is composed by three secondary sliding masses. Generally, the cumulative displacement-time curves are step-shaped, characterized by rapid displacement in the summer followed by almost imperceptible movement at dry seasons. The displacement velocities usually increase spatially with the rise of elevation. The deformation of the Outang landslide is primarily controlled by reservoir water and rainfall. The deformation of the lower part of the landslide is mainly affected by reservoir water, while the deformation of the middle and upper part of the landslide is mainly affected by rainfall. Numerical analyses further reveals that the main controlling factors influencing the pore water pressure of landslide vary with the change of the elevation. The coupled actions of increased in seepage pressure at the front and the pore water pressure at the middle and upper parts result in the reactivation of the Outang landslide. In addition, the slope stability will be reduced to different degrees when the water level of the reservoir decreases or the rainfall increases. The results are very useful not only in the practical engineering but also in the theoretical study of the ancient landslide in reservoir areas. Moreover, strengthening the study of old landslide would enrich the methods on landslide forecast and its stability evaluation, and provide the guideline of ancient landslide prevention.