Abstract: The real-time simulation of reactor coolant system is studied in the paper.Under the real-time simulation support system ASCA,a reactor coolant system is modeled,programmed and computed.The in-fluence of uneven flow between the gas and liquid is calculated with the drift-flux model.The four quadrant homologous curves in term of the dimensionless similarity parameters are used to characterize the pump per-formance in the program.The dynamic trend of system pressure,SG pressure and water level are obtained under the decreasing power in the simulation system.The results show that the program can be used for train-ing simulator and safety analysis of coolant system.