According to the theory of operational reliability, entropy and risk assessment, this paper proposes an assessment method for catastrophic accident of power system based on the current operation state and the importance of component comprehensively. This method uses component’s operational reliability model that considering the cumulative effect to get it’s real-time failure probability of catastrophic accident. Then combine potential energy with entropy to get it’s structural importance that considering active power, reactive power and other factors. Finally a catastrophic accident risk index is built which shows the process from cascading failure to catastrophic accident. Simulation results on the IEEE 57-bus system prove that this method can identify the risks within the system effectively.