Abstract: Based on the modeling experience of CEFR thermal-hydraulics model, the main system model of CFR is established through Relap code. According to the Anticipated Transient without Scram (ATWS) in the safety analysis of the fast reactor, the safety margin and shutdown protection in the event of reactivity accident introduction are studied. The results show that when the accident occurred at rated power, the alarm and shutdown signal of short-period was not triggered. Besides, the current setting value of each protection parameters in the primary loop, the signal measurement delay and drop time of scram rod can be taken from other values, when the reactivity insertion accident of uncontrolled shim rod withdrawal for 5 s or 10 s occurs. While for 15 s, the power and P/F signal can ensure that the reactor status meets the requirements of the accident acceptance criteria in the current design. To achieve the same function for shutdown protection triggered by sodium temperature at core outlet, it is necessary to make sure that the reactor is in depth subcritical before 14.85 s when the other protection signals fail.