Abstract: To ensure and enhance the safety of fast reactor, by comparing and analyzing the existing passive shutdown devices, a new design concept of the melting-alloy passive shutdown system for sodium-cooled fast reactor was proposed based on taking the sudden change of tensile strength of some alloys at certain temperature as the triggering condition for passive shutdown system. This system could bring negative reactivity to the core when unprotected transient-overpower accident or unprotected loss-of-flow accident occurred. A design scheme of melting-alloy passive shutdown system was completed based on China Experimental Fast Reactor (CEFR). The response of the passive shutdown system under the unprotected accident of CEFR was simulated by using DYN4G code and the key parameters of subassembly design for passive shutdown system were confirmed. The results show that during the unprotected accident, a melting alloy passive shutdown system can effectively reduce temperature of fuel subassembly and coolant, and improve the ability of sodium-cooled fast reactor to deal with severe accident.