An analytical model is presented for electromagnetic field distribution within an apertured shielding enclosure. The highlight of this model is that it contains very simple and close expressions of both the electrical field and magnetic field distributions. The inner space of an apertured shielding enclosure is divided into three regions: the region around the aperture is called the aperture region, the part that is close to the backboard can be defined as the waveguide region, and the space between the aperture region and the waveguide region is described as the dipole region. The electromagnetic field distribution model of the aperture region can be replaced by an infinite perfect electric conducting plane with the same aperture. The electromagnetic field distribution model of the dipole region is described by using the dipole radiation model of Bethe’s small aperture coupling theory. The electromagnetic field distribution model of the waveguide region is approximated by using the distribution of the waveguide mode field structure. This model is applied to an investigation of the shielding effectiveness of a rectangular shielding enclosure with a circle aperture under the illumination of a plane wave. The calculated results are in good agreement with those of a full-wave simulation.