High-intensity electromagnetic pulses like high-altitude electromagnetic pulse (HEMP), high power microwave (HPM) are very harmful to electronic devices, which will cause malfunction or even damage to electronics. Using metallic cavity to isolate electronic systems from high-intensity electromagnetic pulse is an important and effective way to lessen the effect. Herein, we performed a systematic study on coupling property of metallic cavity with a rectangular aperture under the excitation of three types of high-intensity electromagnetic pulses, i.e., HEMP, wide-band HPM, and narrow-band HPM. The width/length ratio of aperture and cavity size were varied to investigate their effect on electromagnetic coupling. The interaction mechanism of high-intensity electromagnetic pulses and the metallic cavity was also carefully analyzed. The results demonstrate that the shielding property of metallic cavity is highly dependent on resonance mode of cavity, resonant frequency of aperture as well as the spectral characteristics of high-intensity electromagnetic pulse. When the resonance mode of cavity, and/or resonant frequency of aperture are within the bandwidth of high-intensity electromagnetic pulse, an enlarged coupling electromagnetic field in the metallic is always produced. Specifically, the interaction between the cavity and aperture can give rise to spectral shift of resonant frequency. Therefore, to obtain excellent shielding effect, we need to take into consideration the suppression of resonance mode of cavity, resonant frequency of apertureas well as the spectral shift when performing electromagnetic protection of electronic devices.