Stability issues of high slope in reservoir area encountered frequently during the construction process of water conservancy and hydropower project directly determine the feasibility, construction investment and safe operation of the engineering. Pre field geological survey is the critical link in stability analysis of high slope. Taking a valley landslide mass in reservoir area of certain hydraulic project of Yalong River for instance, by means of field survey and geological processes analysis, the paper analyzed the zone characteristics and sliding mode of the landslide mass. The Results Indicate that the landslide mass remains stable in current native state。But tensile-sliding failure may happen in disadvantage conditions when unloading joints connect in deep part.