Abstract: Hydropower station is generally associated with large scale, fast construction and unforeseen geological problems. Especially the deep anti-sliding stability problem is very important for the safe operation of hydropower project. For the above reasons, this paper takes Guanyinyan hydropower station on JinSha river as example. On the basis of one year work of tracing dam foundation excavation, we develop the observational based design method. The method including the following steps: the engineering geological conditions, the observational evaluation of weak intercalation characteristics, the observation based parameter values of weak intercalated structures, and the stability of the dam foundation. It takes the observational analysis of the weak intercalation distribution characteristics and its parameter as the core. The reasonable deep anti-sliding stability evaluation method is set up for dam foundation. The practice shows that the set of methods ensure the hydropower project construction period, and also provide reliable data for the stability evaluation.