Abstract: Based on the simple introduction of recent development status of the long-life small nuclear burnup and a deep burnup core.The Program MCNP and MCBurn are used in the calculation of a Pin cell model with different initial driver fuels,p/d ratios,enrichments and fuel types to obtain some results for ex-power systems in the world,this paper puts forward a new concept of building a long-life reactor core with Thorium-Uranium fuel and Lead-Bismuth coolant and conducts a series of research on the characteristics of the Th-U fuel.In the physics design of a long-life reactor,it is important to keep a small reactivity swing with ample neutron spectrum,the reactivity swing with burnup and initial conversion ratio,etc.On the analysis of these results,this paper concludes some physical requirements of a long-life reactor core and constructs a core design with some safety parameters such as void reactivity coefficient and other results.