We propose a four-channel plasmonic demultiplexer based on gold–air–gold structure. To do so, an add-drop filter is first proposed consisting of an H-shaped resonant cavity, which is designed to drop a distinct wavelength to other waveguides. Since the selected wavelength depends on the structural parameters of the resonant cavity, two- and four-channel demultiplexers are presented by adding H-shape cavities with different sizes. The results of numerical simulations show that for the total structure, spacing about 50 nm and an average transmission more than 95% for channels are obtained. The proposed device exhibits high-transmission efficiency along with the low channel spacing. The average calculated cross talk is calculated about .