Abstract: Small natural circulation lead-cooled fast reactor has a good property of passive safety and economic. The primary cooling system model was established, based on the concept of a 100 MW_th small natural circulation lead-cooled fast reactor（SNCLFR-100）. With RELAP5 code, initial state and transient conditions were analyzed, including the protected transient of overpower and loss of heat sink, and the unprotected transient of overpower and loss of heat sink. The results show that during the protected transient of overpower and loss of heat sink process, the reactor is safe with the help of reactor trip protection system. As for the unprotected one, the reactor shuts itself down on account of the strong negative feedback of reactivity in 500 s, and the temperatures of coolant, cladding and fuel pellet are below the safety limits. The simulations of the two transient could verity the inherent safety of this new reactor.