Abstract: 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.