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Since electric components and printed circuit board in the enclosure can be destroyed by electromagnetic pulse weapons through “front door and back door” coupling, which is a great threat to the operational security, the study of the shielding effectiveness is of important significance. A formulation for shielding effectiveness analysis of a rectangular enclosure with an electrically large aperture is proposed in this paper. Firstly, the plane wave with oblique incidence and polarization is decomposed. Secondly, based on the Cohn model, the equivalent electric and magnetic dipole of the electrically large aperture is computed. Thirdly, the total Hertz electric and magnetic vector potential is obtained through mirror procedure. Finally, the electric field inside an enclosure with electrically large aperture is formulated, which is used for shielding effectiveness calculation. Five verification experiments are designed. Simulation result shows that the mean square error and absolute error of this method compared to computer simulation technology (CST) microwave studio are 11.565 dB and 8.015 dB respectively, the correlation coefficient is 0.921, through which the accuracy of this method is verified. The simulation time of this method is 0.183 s, which is only 1/7530 times of CST, so its efficiency is obvious.