Abstract: Stability of slopes comprising colluvium is very important to the hydropower station construction in the southeast of China especially. The key tasks are to find the answers for the questions: the reasons of accumulation formation, mechanical parameter values and slope stability under different conditions. This paper analyzes the typical engineering geology condition of the dam site zone of a hydropower station at Gushui. The lower zone is slate and the upper is colluvium. The paper examines the history of the engineering geology condition at this region. It further examines the formation of the colluvium at the upper zone and obtains the topping deformation of slate at the lower zone. Due to schlep effect, the colluvium was formed. The rock-soil materials on the top of slope move down in schlep effect of outwash. The colluvium has a clear layering structure. The slate experiences a continuous increase in under the effect of tectonic movement, incised valley and weathering unloading and also the weight of upper colluvium. Breaking of the slate is happened and lead to topping deformation. Based on the analysis of physical and mechanical characteristics of the colluvium, and combined with the survey of engineering geology condition, the mechanism of failure and deformation of the colluvium slope is obtained. Its contains the wholly slide failure along the weakly layer and the locally arc slide failure.