Abstract: This paper introduces the theoretic model for the successive layer coupling method for the core power distribution expanding and the solution method for the successive over relaxation equation. The calculation code EXP for the core power distribution expanding, which is based on these methods, has been programmed in FORTRAN90 computer language. The theoretic and metrical data of the unit 1, cycle 11 of Daya Bay NPP are used for analyzing the sensitivity and validity of the expanding calculation results. The results show that, this expanding method uses the theoretic data to build the coupling connection of assemblies in the core and uses the metrical data to calculate the power of all assemblies, and then the expanding calculation is conducted. The method is correct and effective, and has a good adaptability for the invalidation of the measure points.