The transition of power system operation state directly influences the system security performance analysis and dispatch operation strategy. If the security state of a power system and the damage degree can be known before the state transition， it would be a great help for power system security stability. Based on the forewarning of the security state with assessment indices， the quantitative security assessment indices of five power system operation states according to a risk analysis are proposed， which forewarn the probability of the risk in state transition and support the decision-making on low voltage prevention， overload prevention and system voltage instability prevention. The simulative analysis of an 84-bus system in Shanghai Power Grid shows that the proposed approach proposed is feasible and effective.