The electromagnetic transmission suppression effect in dual-subwavelength metal gratings was studied based on the transmission calculated by the Transfer Matrix Method. Combining with the field distribution property of transmission suppression, an equivalent rectangular cavity model was proposed, and the relationships of the structure parameters and the wavelength were obtained. The transmission suppression effect is irrelevant to the thickness of the composed single grating, which means that, it is independent on the waveguide modes of the single grating, but quite dependent on the evanescent mode coupling. The field distributions show that there is almost zero EM field at the slit entrances of the second grating. The equivalent perfect electric conductor rectangular cavity model provides the analytic expression of the field distribution, and furthermore, the transmission line consistent with the one observed from the transmission spectrum is figured out.
