The construction of hydropower station in west China gave rise to an increasingly obvious problem of environmental slope hazard. The stability of slope within the opening line received sufficient attention and regulation by human engineering activities ； whereas the environmental slope outside the opening line still keeps its natural stability. It is neither necessary nor possible to make comprehensive analysis or extensive regulation on its natural state and to impose the same safety control standard as with excavated slopes. In addition, the lack of standard guidance or research documents published for reference brings about neither systematic nor unified evaluation criteria and protective measures, which has become a potential safety hazard for the construction and operation of hydropower stations. In this paper, issues regarding the defining, classification, slope safety level, prevention and control criterion of environmental slope hazard are expounded, and a risk assessment system is proposed to provide basis for further research.