Accident analysis is an important part of nuclear power plant safety analysis. The purpose of long-term safety and stability analysis after an accident is to ensure that a nuclear power plant can return to a safe state in the event of an accident. In order to research whether the VVER-1200 reactor can enter the long-term safety and stability state after the FLB (feedwater line break) accident, the actions needed to restore the long-term safety and stability of all kinds of reactors are analyzed. The accident process, system action and operator action of FLB accident are researched. The model of VVER-1200 is built by TRAP-97 program, and the FLB accident of VVER-1200 is calculated, the event sequence is obtained. The calculation results show that the parameters of the reactor can meet the requirements of the acceptance criteria, and can enter the long-term safety and stability state under the operator’s action. This research results can provide reference and support for the accident analysis of VVER-1200 and the long-term safety and stability analysis of various reactors.