Abstract: The system analysis software SAC3D was used to model the core and primary loop of the U.S. Fast Flux Test Facility (FFTF) in the paper. The accident transient simulations were carried out based on the boundary condition data of FFTF loss of flow without scram test provided by the International Atomic Energy Agency (IAEA). The thermalhydraulic and neutron physical key parameters of the core were calculated, and the simulation results are in good agreement with the experimental measurement data. The comparison results verify the validity and accuracy of the SAC3D in simulating the accident conditions of liquid metal cooled fast reactors and prove that the FFTF reactor type has reliable passive safety.