Abstract: Removement of residual heat is one of the significant safety functions of a nuclear reactor, and the shutdown residual heat at the beginning is composed of fission power and decay heat, meanwhile the long-term residual heat mainly depends on the decay heat. In this paper, a calculation model for the decay heat of sodium-cooled fast reactor system analysis code FR-Sdaso was developed, which has the ability to take the effects of fission power and power history into consideration. The model was validated by comparing with ANSI/ANS-5.1-2005 standard and SAS4A/SASYS-1 code. The maximum relative deviation between FR-Sdaso and ANSI/ANS-5.1-2005 standard is about 0.1%, and the maximum relative deviation between FR-sdaso and SAS4A/SASYS-1 is on the order of 10-8, which initially proves the correctness of the developed model. Finally, based on the design data of China Experimental Fast Reactor, the effect of fission power on the decay heat during the emergency shutdown process was analyzed. The results show that the maximum relative deviation of the decay heat caused by neglecting fission power is about -7%, which appears at the initial shutdown. Therefore, the effect of fission power should be considered in the calculation of decay heat at the initial shutdown.