Herein, an eight-wavelength photonic-crystal wavelength demultiplexer was designed based on the superior frequency-selection characteristics of a photonic crystal resonator. The energy band structure of the photonic-crystal with a specific lattice constant and radius was obtained using the plane wave expansion method. The law of variation in the coupling frequency of a micro-cavity was analyzed via the finite difference time domain method. The resonant coupling between the ring resonator and the micro-cavity was demonstrated. The device primarily comprised four photonic-crystal ring resonators and eight micro-cavities with different sizes. It achieved wavelength-division multiplexing at eight wavelengths of 1.37, 1.39, 1.42, 1.44, 1.50, 1.51, 1.53, and 1.55 μm. The results show that the aforementioned eight wavelengths can be output from a specific port, and an output efficiency of more than 97% can be achieved by adjusting the radii of the central rods of the micro-cavity and the peripheral dielectric rods. The designed device can be applied in integrated optics owing to its small physical size of 23 μm×18 μm and high coupling efficiency.