Abstract: This paper presented a codes coupling method of system analysis code and integrated severe accident (SA) code to develop a new SA simulator. It uses system analysis code to calculate early stage of an accident, when satisfying coupling condition, system analysis code stops calculation and switches to SA code to simulate mid-late-term accident. In order to achieve smooth transition of parameters, this paper took full scope simulator general code RELAP5 and SA code MAAP4 as example to analyze their repetitive thermal-hydraulic models, especially the core model. Core fuel temperature, cladding temperature and core power were selected as transition parameters. Based on a same small break loss of coolant accident (SBLOCA) of generic million kilowatts of Pressurized Water Reactor (PWR), coupling transition calculation and SA code independent calculation were compared to test the method. The simulation results showed that this selection of transition parameters is correct. This method can not only smoothly transmit the parameters but also ensure accuracy of subsequent calculation.