A wavelength-division demultiplexer based on two-dimensional triangular lattice photonic crystals is proposed. This structure is composed of line-defect waveguides, ring resonators and point-defect nanocavities. The characteristics of line-defect waveguide are investigated via the method of the plane-wave expansion, and the projected band diagrams of line-defect waveguide are derived. Then appropriate parameters are obtained after a lot of simulations of the fine tuning of local components and the whole device. The transmission characteristics of light with different wavelengths in the photonic crystals demultiplexer are analyzed via the finite-difference time-domain (FDTD) method, and the resulting electric fields are also given. The results show the ability of wavelength-division demultiplexing between 1271 nm and 1291 nm with this device. Besides, six extra dielectric rods are added to enhance the transmission of the ring cavity, and by adding three pairs of dielectric rods at the entrance port, the total output efficiency of the system is increased.