Abstract: In order to meet the modular design requirements of sodium-cooled small modular reactor, a passive decay heat removal system solution based on natural circulation of air within the containment was proposed. Through theoretical calculation and system analysis program RELAP5, the passive decay heat removal system was modeled and the feasibility of the system scheme was analyzed. The results show that under the conservative assumption, when the sodium loading is 2000 kg, the temperature of core fuel will not exceed the safety limits when the thermal power of passive decay heat removal system is more than 16.88 kW. The calculation results of RELAP5 show that the cooling power of the passive decay heat removal system is larger than the minimum power required, and can meet the design requirements of sodium-cooled small modular reactor.