Hazard level of potentially hazardous rock mass(PHRM) is referred to as activity, characteristics, geographic distribution and their effect on objects suffered hazards. It is relevant not only to the stability of PHRM,but also to object, height above object and size of PHRM. Under given other factors, with the increase of size and height, the hazard level increases. Through studying surrounding slope of hydroelectric projects finished or proposed, we establish a classification system for evaluating the stability and risk level of PHRM. On the basis of the system, we present the suitable prevention measures firstly, risk level of object influenced by PHRM is classified according to level and position of hydraulic structures in association with density of surrounding people; then, we classify hazard levels of PHRM in view of the stability and size of PHRM and height above object. This method is applied to evaluation and treatment of PHRM in the surrounding slope of most hydroelectric projects of our corporation, and results show that it is effective.