Recently　most　of　the　high-fidelity　reactor　physics　calculation　used　the　characteristics　based　2D/1D　coupling　method　as　a　neutron　transport　solver.　In　the　classical　2D/1D　method,　the　accuracy　of　the　axial　leakage　will　have　direct　effect　on　the　final　calculation　precision.　In　order　to　obtain　more　accurate　axial　leakage,　the　2D/3D　coupling　method　employing　the　global　3D　SN　method　to　get　the　axial　leakage　for　2D　MOC　calculation,　meanwhile　the　2D　MOC　calculation　supplies　the　homogenization　cross　section　for　3D　SN　calculation.　To　make　sure　that　the　3D　SN　calculation　can　take　consideration　of　the　space　distribution　of　the　flux　in　each　pin　cell,　a　flux　correct　factor　is　introduced　when　computing　the　outgoing　angular　flux　in　3D　SN　calculation.　In　2D/3D　coupling　method,　iteration　between　the　2D　MOC　and　3D　SN　is　implemented　to　improve　the　accuracy.　In　the　paper,　the　detailed　theory　of　this　2D/3D　coupling　method　is　introduced,　which　includes　the　derivation　of　the　coupling　equations　and　the　iterative　calculation　flow,　then　the　impact　of　axial　leakage　to　the　transport　results　is　analyzed.　Based　on　the　theory　a　2D/3D　coupling　code　is　developed　and　the　results　of　the　3D　C5G7　problems　indicates　that　with　less　MOC　calculation　layers　the　2D/3D　coupling　method　can　obtain　accuracy　solution　and　be　able　to　solve　small　whole-core　transport　problem.　©　2018,　Editorial　Board　of　Journal　of　Nuclear　Power　Engineering.　All　right　reserved.