The ring resonator is the basic functional unit in a photonic crystal wavelength-division multiplexer (WDM). It can realize the separation and multiplexing of optical signals of different wavelengths. Optical signal transmission can be simultaneously enhanced both in capacity and efficiency by using WDMs. A 2D-3D heterostructure is used as the basis of this work, which allows dense wavelength-division multiplexing. A ring resonator is introduced into the two-dimensional photonic crystal plane. Wavelength-division multiplexing is then realized by changing the cavity length of the resonator and the number of internal dielectric columns.