Abstract: An investigation of nuclear reactor simulator was made on its physics model, thermal hydraulics model, modeling method for assistant system and numerical calculation technology. The simultaneous equations for each module and node of the steam supply system are solved by full implicit difference method. Then the supporting and calculation softwares for simulation was developed based on Windows system. Calculation and comparison have been done for small break LOCA by the safety analysis simulator developed in this paper and Relap5 code. The results show that the variation of primary parameters for break LOCA of this model accord with the calculation results from Relap5, and the simulator can perform real time calculation with well stability.