Abstract: In order to study the unsymmetrical operating condition of China Experimental Fast Reactor (CEFR), the temperature and flow rate of CEFR operating with only single loop were calculated with the system analysis code OASIS and the sub-channel code COBRA. The results show that when CEFR is operating with only single loop while the rotating speeds of the primary sodium pump in that loop and the sodium pump in the corresponding secondary loop are both at 500 r/min and the check valve of the other primary pump of the failed loop is open, the max power of the core should be set at 14% in order to ensure that the reactor is safe.