A wave demultiplexer system with channels in a two-dimensional photonic crystal is proposed. The demultiplexer is realized by the coupling among an ultra-low-quality factor microcavity and resonators with high-quality factor. The coupling mode theory is employed to analyze the behavior of this system. The analytic results reveal that the reflection is fully absent at the peak frequencies for all channels. The simulations obtained by multiple-scattering method and experimental results in the microwave region show that the analysis is valid. This method might also be valuable for the design of other all-optical functional circuits.