Abstract: Aiming at prediction for the electromagnetic coupling characteristics of a metallic enclosure with a rectangular aperture, this paper proposes a hybrid approach which applies the mode-matching technique and the mixed potential integral equation based on the method of moments. The effects of high-order modes, aperture shape, aperture thickness, and polarization direction of incident wave are all considered in the simulation. The simulation results are compared with data obtained by measurement and finite-difference time-domain method to validate the accuracy and the efficiency of the proposed approach. The research shows that there is an obvious resonance when electromagnetic waves penetrating the metallic enclosure with a rectangular aperture; the coupling electric fields around the aperture and center of the enclosure are enhanced at the resonant frequency; with the thickness of the apertures growing, the coupling electric fields weaken; when the polarized direction of the incident wave is parallel to the short side of the rectangular aperture, the coupled electric fields attain the maximum.