Abstract: The numerical simulation models of local break and the entire system under loss of coolant accident（LOCA） for a certain gas pressurized research reactor are established. Based on the simulation of main pipe break, the relations among system flowrate, system pressure and break flowrate are studied, and the resistance characteristics of the break is obtained. Coupled the characteristic data with the system simulation model, the two schemes, isolation and depressurization, to prevent the gas into the reactor under LOCA accident are studied. The results show that, the break flowrate mainly depends on the system pressure, and the depressurization scheme is better than the isolation scheme.