Based on the Bethe's theory for small aperture coupling and the eigen-mode expansion method, an approximate analytic model for the field distribution inside a rectangular cavity with apertures under plane wave illumination is presented. The effects of aperture shape, dimensions, number, position, and the wave incidence and polarization angles on cavity mode excitation can be taken into consideration in this model with clear physical explanation. The calculation results from this model are in better agreement with experimental data than those from the equivalent circuit model. The effects of various factors on the shielding effectiveness of the cavity are analyzed, and the obtained results are useful for guiding the design of electromagnetic shielding enclosures.