Abstract: The conventional methods used for system risk analysis are static in nature,such as event tree or fault tree method.To describe the system state variation,we must take multiple factors into account,which include time ,procedure variables,hardware,historical events and human factor.For this purpose,GO-FLOW was applied to the reliability analysis of the nuclear reactor purification system with obvious dynamic characteristics.GO-FLOW dynamic model of standby unit was improved,and established system GO-FLOW chart has effectively modeled the changing process of system＇s structure and configuration,as well as the operator＇s quality state and behavior under initial event.The analysis results show that the probability of the purification system failure changes significantly and the intervention by the operator can effectively mitigate the system risk due to hardwa re failure.It is also verified that the GO-FLOW methodology is an effective and useful analysis tool for dynamic system.