Abstract: System analysis code for China fast reactor (SAC-CFR) which was a self-developed computer code was used to analyze whole-plant transient behavior of the Experimental Breeder Reactor Ⅱ (EBR-Ⅱ) under loss of heat sink without scram (LOHSWS) accident. The three-dimensional hot pool model coupled in SAC-CFR was used to analyze flow patterns of the flow filled in EBR-Ⅱ. The results show that the predicted results agree well with test data, and SAC-CFR can be used to calculate transient behavior under some accident of fast reactor. At the same time, it is concluded that EBR-Ⅱ can shut down itself by inherent safety under LOHSWS accident.