Abstract: The design objectives of a long life reactor include the high fuel burnup and full power natural circulation,which increases the difficulty of reactor physical design: a higher conversion ratio to achieve zero reactivity swing with burnup and increase the P/D ratio to achieve natural circulation.It is a hard mission for the U-Pu fuel long life reactor design.This paper describes a spent Pu-Thorium fuel and lead-bismuth coolant based long life core design,taking the advantages of thorium fuel in fast neutron spectrum.The MCBurn code is chosen to perform the physical calculations..