Abstract: The passive core cooling system (PXS) was applied to fusion-fission hybrid reactor and a RELAP5 model was developed to represent the primary loop, partial secondary loop and the PXS components. The transient calculation and analysis were conducted for the double ended rupture LBLOCA on cold leg. The results show that two peak fuel temperatures appear in the outer blanket during the transient duration after the break happens. The first peak fuel temperature is 938.2 K at about 11 s and the second is 608.7 K at about 50 s, but both are below the melting point of U-10Zr and within the acceptable range. With the transient process developing, the coolant in ACC, CMT and IWST was injected to the blanket and the collapsed liquid level in the blanket started to rise, and finally the reactor core was covered by injected coolant again. The results indicate that the PXS can ensure the safety of fusion-fission hybrid reactor in the double ended rupture LBLOCA on cold leg and it is feasible to apply the PXS to fusion-fission hybrid reactor.