Abstract: Based on GO-FLOW methodology, the reliability of emergency core cooling system （ECCS） in lose of coolant accident （LOCA） was analyzed. GO-FLOW chart was drawn according to the principle chart of ECCS, and system success rate at each time point was calculated with GO-FLOW algorithm. The results show that the reliability of ECCS is changed acutely in the process of LOCA, and GO-FLOW methodology can calculate the reliability change tendency and it is an effective way for solving multiple operating sequence and multiple states problems in reliability analysis.