In order to investigate the influence of an opening in a metal cavity on the destruction efficiency of HEMP (high-amplitude electromagnetic pulse) and HPM (high power microwave), the model of electromagnetic pulse coupling into an open-ended metal enclosure was built with the help of CST software. Based on the model, the influences of the location, size and length-to-width ratio of the aperture on coupling effects of HEMP and HPM were researched. As for HEMP, the location, size and length-to-width ratio of the aperture can exert great influence on the coupling effect, so the destruction efficiency of HEMP can be weakened by reasonable controls of these factors. As for HPM, its coupling effect is obviously stronger than that of HEMP under the same conditions. After the aperture reaches a certain size, its size and length-to-width ratio have few influences on the coupling effect of HPM, and only the location of the aperture has a great influence. The coupling effect is the weakest when the plane of the opening is in parallel with the incident direction of HPM, but the energy coupled into the cavity can easily reach the electromagnetic damage level of many electronic components.