Most of the current methods for safety analysis belong to the scope of safety pre-analyzing, which analyzes the system safety before system being in service. But the system safety is full of variety because of the system's dynamic characteristic, such as the running status variation of system component. In order to know well the real-time safety risk, a real-time monitoring method for system safety is proposed. The safety risk model was established on the fault tree that was used to represent logic structure of system. The real-time safety risk was monitored according to the correspondence between technical status of system component and running status of system component. Simultaneously the method could account for the change of risks, thus ensuing the taking of precautions. Finally a case monitoring the safety risk for cooling system with pre-feedback was used to demonstrate the effectiveness of the method.