Safety is an issue that must be considered in the development of sodium cooled fast reactors, severe accident evaluation plays an important role in fast reactor safety analysis. SAS4A program was selected as the analysis tool to systematically consider the main reactivity changes in the initial stage of severe accident, and it is applied to the analysis and calculation of sodium-cooled fast reactor unprotected loss of flow (ULOF) accident. The calculations show that in the early stage of the ULOF accident of the sodium-cooled fast reactor, the negative reactivity introduced by the fuel temperature rise and the axial expansion of the fuel assembly make the reactor power slowly declines. Then, due to the increase of coolant temperature, coolant in some areas began to boil, which caused the density of the coolant to decrease rapidly. The core introduced a large amount of positive reactivity and the power increased rapidly, then a power peak appeared. Subsequently, the power increased and decreased due to the movement of the molten cladding and the fuel, respectively. Finally, due to the large amount of molten cladding and fuel movement, the severe accident of the sodium-cooled fast reactor entered the transition phase.