A novel 1.31/1.55 μm wavelength-division demultiplexer based on selectively liquid-filled three-core photonic crystal fibers (PCFs) is proposed. It consists of a normal central silica core and other two cores selectively filled with different refractive indices of liquid materials. According to coupling mode theory, there exists wavelength-selective coupling of evanescent fields between nonidentical single-mode fibers. Two optial fibers with different response mavelength are set up respectively when two cores filled with different materials couple with the central core at two particular wavelengths. There are two corresponding wavelengths in three-core PCFs and then two beams of light with different wavelengths can be separated from each other if appropriate parameters are chosen. Full-vectorial finite element method (FEM) is employed to analyze the properties of PCFs and simulate its coupling wavelength and length with different filled refractive indices. Numerical results by beam propagation method (BPM) demonstrate that it is possible to obtain a 4.88 mm-long 1.31/1.55 μm wavelength-division demultiplexer.