Abstract: This paper builds a primary loop thermal-hydraulic calculation model of High Flux Engineering Test Reactor（HFETR） based on Relap5, and the model is proved to be effective and accurate by an experiment in cold state. By simulating the full open fault of degassing flow control valve（DFCV） happened in running state with this model, the varieties of inlet pressure, outlet pressure and water level of volume compensator are obtained. The results of modeling show that fuel rods of HFETR are confirmed to be well submerged and cooled down, and HFETR remains safe. The reaction time for reactor operators and the reasonable water level of volume compensator are given by analyzing the calculation results, and an advice on DFCV working sate adjustment is proposed.