Abstract: Based on the theory of thermal hydraulic calculation of RELAP5, the application of the coupled parallel technology by heat flux and wall temperature in the full scope simulator of nuclear power plants is studied. The model splits up into two coupling parallel computing models by wall temperature or heat flux of the heat structure. Comparing the results with the overall calculation results, it shows that: the results of coupling calculation by heat flux of recuperative heat exchanger are inaccuracy; and the results of coupling calculation by wall temperature are consistent with the actual value. The simulation results of the steam supply system of the typical nuclear power plant by using coupling parallel computing model by wall temperature shows that: the coupling parallel computing by wall temperature of recuperative heat exchanger effectively improves the CPU utilization and the computation speed, which provides more guarantee for the real-time calculation of the simulation.