Abstract: In order to study the safety margin in the current reactor design, the thermal hydraulics model of CEFR, sodium cooled fast reactor, is established in simulink platform. The simulation analysis of the reactor state when the flow of the primary loop changes is executed in the manner of anticipated transient without scram used in the safety analysis in fast reactor.And ensure that the state of the sodium-cooled fast reactor meets the requirements of the accident acceptance criteria for sodium-cooled fast reactors during the entire accident process. The results shows only the P/F and the sodium temperature at core outlet signal are triggered in the entire process when the flow changes. And compared to the status after the sodium temperature at core outlet is triggered, the reactor enters into depth subcritical earlier if the P/F is triggered.