Cascading outage is the main cause leading to Black Out. This paper described a power system operation risk assessment method based on possibility prioritization search strategy for reducing the risk of power system cascading outage. The proposed method describe the power system cascading outage as a determinate dynamic process triggered by an occasional event, and from this point of view the risk assessment theory is introduced in estimation of power system operation in order to produce the cascading outage risk indices reflecting security level of power system.