Resonant cavities are used widely in the field of integrated optics due to their excellent frequency-selective properties. Based on the characteristics of resonant coupling between two photonic crystal ring resonators, four cavities with different sizes and waveguides, the demultiplexing of four wavelengths, i.e. 1310 nm, 1550 nm, 1600 nm and 1650 nm is realized. The numerical results obtained by the finite-difference time-domain method show that the output efficiency of the four wavelengths can be higher than 90% only by modulating the radii of the border rods of output waveguides. The presented device that has not only a compact size with 12 μ m× 17μm but also a high output efficiency, may have potential application in the future optical communication field.