Located at the right bank of the reservoir near the dam site of Shiziping hydroelectric station, the studied deposit landslide is a gradually destroyed retrograde landslide caused by the collapse deposit under the effect of reservoir water along the interface between deposit and bedrock. The first impoundment of the reservoir in September 2009 leads to intense deformation and failure of the collapse deposit, hence causes great harm to the 317 national highway. In this sense, it is of great significance to study the formation mechanism of the collapse deposit so as to prevent similar landslides from happening again. According to the geological condition, the structure of the slidemass, and the deformation evidence of the slope during reservoir impoundment, the deformation mechanism is studied, and the stability of the slidemass is evaluated and predicted. Results show that when the water reaches the natural level of 2 490 m , the slidemass will creep ； when there is rainstorm, earthquake, or when the water reaches the normal impoundment level of 2 540m , failure of the slidemass will occur. Currently the deposit landslide is relatively stable after proper emergency measures were taken.