Abstract: An opening rectangular box model is proposed to calculate electromagnetic energy coupled to the center of model under the plane wave irradiation from outside. First of all, according to the ideal conductor surface boundary conditions, electric field integral equation (EFIE) of the scattering field in three-dimensional space is deduced. Then the surface induction current can be expanded by using of RWG base function, which is verified by galerkin method. In the end, spread coefficient of surface current can solved by moment method to obtain induced current distribution on surface of whole box. By using of EFIE and FEKO, the shielding effectiveness under different conditions, such as the single hole, the multiple holes and the round hole, is calculated. The results show that more holes will make better shielding effectiveness under constraint condition of same area, and the shielding effectiveness of round hole is better than that of rectangle hole.