Abstract: In the safety assessment of High Flux Engineering Test Reactor（HFETR）, the Loss of-Coolant Accident（LOCA） is one of the most important initial events. This paper establishes a numerical calculation model of HFETR reactor based on RELAP5, and simulates the testing case of LOCA. By opening the DN50 valve of primary loop degassing system, the LOCA was simulated. The parameters of reactor inlet and outlet pressures, regulator pressure, and breaking flow rate were tested. Meanwhile, through comparison and analysis of testing data and calculation data, the rationality of the calculation result of RELAP5 program was evaluated. The results show that the RELAP5 calculation result is in good agreement with the experimental results, and the maximum relative error is 7.4%. Those results imply that it is feasible to use the RELAP5 program to simulate the system transients at a low-pressure, pressurized water reactor, such as HFETR.