Abstract: Based on detailed field geology survey, this paper examines the Cimodi ancient landslide in Yalongjiang River and evaluates its stability. The macro geological conditions are obtained from the aspects of topography and geomorphology, historical earthquake records and material structure and other geological conditions. A conclusion is obtained that in natural state, the ancient landslide is stable and the slope stability is up to original sliding soil and rock mass interface and potential sliding interface in the slope body. Refer to the inversion analysis of the morphological changes of ancient landslide before and after, according to the energy method, landslide kinematics and Scheideg-gar hypothesis, this paper determines four key landslide motion parameters: landslide radius r, equivalent slope degree ,integrated dynamic friction angle and trailing gap Hm. The max landslide speed Vmax can be calculated with landslide motion equation. Specially, there are two methods to value the trailing gap Hm: One method is considered that the tailing gap point can stop movement when the max landslide speed Vmax occurs and the Hm of Vmax is the vertical distance of trailing gap point before and after the sliding movement. The other method is considered that the tailing gap point can keep moving together with the slope body and slow down after the occurrence of the Vmax. Therefore the Hm of Vmax, which is less than the vertical distance of trailing gap point before and after the sliding movement, could be solved by circular arc sliding surface hypothesis and sliding movement formula. Finally, this paper suggests that ancient landslide is a high speed landslide and the max landslide speed Vmax value should be 13.50ms-1～21.20ms-1. It discusses the characteristics of geology condition of the ancient landslide with high speed. It is a meaningful reference to those unstable slopes in similar engineering geology background in the area nearby Cimodi ancient landslide.