Abstract: The project is a part of STC partnership of CEA-CAEA Co-operation Protocol in the peaceful uses of nuclear energy.This paper mainly presents BUC（BURNUP CREDIT） method and related computer code package（APOLLO-2,DARWIN and CRISTAL）.By using these codes,sensitivity analysis for main physics parameters（including fuel temperature,moderator temperature,critical boron concentration and etc.） of fuel assembly in different irradiation conditions is calculated.The result shows that the neutron multiplica-tion factor will increase as the increasing of temperature and boron concentration.In order to consider the BUC method conservatively,the isotopic correction factor and its calculation uncertainty are taken into ac-count.Then,the paper lists the comparison of the results between calculation and experiment.Finally,this paper gives the analysis of BUC method according to five different axial burnup profiles.