Abstract: Ancient landslides are the product of long-term complex evolution of slopes. Due to intensification of human engineering activities and the impact of earthquakes and heavy rainfall, the problem of ancient landslides reactivation has become increasingly prominent. The damage caused by reactivation of landslide is particularly serious as the detritus soil ancient landslides contain a lot of gravel and rock debris. Taking the Chirong detritus soil ancient landslide in Garzi Prefecture in southwest China as the research object, this paper analyses the reactivation evolution mechanism of the detritus soil ancient landslide from the perspective of engineering geology. The external factors such as topography, geological structure, earthquake and rainfall etc.are systematically studied, and the physical properties and permeability of the detritus soil ancient landslide and slippery soil are analyzed. According to research and analysis, the results are concluded:① The ancient landslide belongs to the multi-period reactivation landslide, which is roughly divided into three stages: ancient, old and new according to the order of landslide formation;② The seepage pattern of detritus soil landslide that coarse gravels form skeleton and rainfall infiltration channel is the main external factor causing the reactivation of detritus soil ancient landslide；③ The reactivation evolution process of the detritus soil ancient landslide is summarized as "four-period, six-process" through engineering geological analysis.