Abstract: A calculation method for the shielding effectiveness of an enclosure with an oblique rectangular aperture under complex interference is proposed. This method can quickly and accurately calculate the shielding efficiency of different metal material , off-centered apertures, off-centered observation point position, oblique incidence and polarization and oblique aperture array of enclosures. This enclosure is first decomposed into two ones with horizontal rectangular apertures through mirror image and magnetic flow theories . Then the shielding effectiveness of each decomposed enclosure is solved based on Robinson ′s equivalent circuit method. The shielding effectiveness of the metal enclosure with an oblique aperture can be obtained by using the above results to convert through the formula. Finally, the calculation method of shielding effectiveness under different conditions is given for the different interferences that the oblique aperture metal enclosure will face. Considering the complex situation of multiple interference superpositions, by programming the method in Matlab, the results are compared with the simulation results of the time-domain transmission line matrix method and the frequency-domain finite element method in the full-wave simulation software CST. The feasibility of the method in this paper is verified by the comparison results. The method shows great advantages in calculation efficiency while ensuring the calculation accuracy.