Abstract: In order to expanding the scope of the full scale simulator (FSS) to severe accident, a simulation code was developed and named SimSA, and the main processes of severe accident can be modeled in this code. There are three main modules in the SimSA code, including thermal-hydraulics module (Therm), core behavior module (Core) and containment module (Cont). A multi-physical coupling method similar to SCDAP/RELAP5 was used in the integration between Therm and Core. This paper introduced the application process of the multi-physical coupling method in the development of SimSA, and it was used to calculate and analyze the severe accident sequences of loss of coolant (LOCA) with failure of safety injection and station blackout (SBO) with loss of auxiliary feed water (AFW). The calculation results of this code were compared with the calculation results of MAAP4 code. The results indicate that the application of the multi-physical coupling method in SimSA is successful.