According to three-wavelength-division-multiplexer, a three-port all-fiber optical Wavelength-divisionmultiplexer was developed by using a new configuration of cascading three 3×3 directional fiber couplers, which was made of three 3×3 single-mode fiber couplers connected by 3 single-mode fibers. Based on the principle of characteristics of fiber transmissivity, the complex amplitude output spectral expression was established and described by using the matrix transfer function. Through numerical simulation and experiment, the influence of the coupling coefficient of the couplers and the lengths of these 3 single-mode fiber on the three-wavelength-division-multiplexer was analyzed. The analytical results indicate that when the lengths of these 3 single-mode fiber are L, L+ΔL, and L+ 2ΔL, respectively and the coupling coefficient of the couplers is some certain values, three-wavelength-division-multiplexer acts as a 3 channel wavelength-division-multiplexer and the port output curves are quite ideal. The experimental sample is tested at 1.55 μm working wavelength and the peak output power interval of 0.8 nm. The experiment results are in good agreement with the analytical ones.