In order to realize the function of wavelength division multiplexing(WDM) device with compact structure and high transmittance, a ψ-shape three-channel wavelength division multiplexing device is designed on the triangle lattice photonic crystals which are composed of Si medium columns. Defect modes are produced in the photonic band gap by the introduction of line defects a point defects, the propagation and coupling of light in the photonic crystals are controlled by taking advantage of the defect modes, and the function of solution WDM are thus realized. The band gap and transmission characteristic of the solution WDM were analysed using plane-wave expansion and finite deference time domain method. The parameters of photonic crystal structure with proper defect mode frequency were found and the function of solution WDM with transmittance of 80% was realized.