Abstract: In the flow reverse of High Flux Engineering Test Reactor (HFETR), the change of the reactor core’s thermal parameters affects the safety of the reactor operation. So the model of HFETR by the RELAP5/MOD3 code was established and relevant research was conducted. The study results indicate that the maximum permissible power for the conversion from forced circulation to natural circulation is 850 kW, the time for the conversion from natural circulation to forced circulation is decided by the upper coolant temperature. The results show that the recent operating mode can guarantee the safe operation of the reactor, and the data provide the support for the more perfect operation mode in the future.