An 1×4 Optical Multiplexer (OMUX) based on the self-collimation effect of a two-dimensional photonic crystal was proposed and its performance was numerically demonstrated. Two Mach-Zehnder Interferometers(MZIs) with different cavity lengths were placed in the structure. Firstly, the theoretical transmission spectra at different output ports of the cascaded MZI were analyzed with the theory of light interference, then they were investigated with the Finite-difference Time-domain (FDTD) simulation technique. The simulation results agree well with the theoretical prediction, so the cascaded Mach-Zehnder interferometer can work as a 1×4 optical multiplexer. When the wavelength is 1 550 nm, the free spectral range of the OMUX is about 34.1 nm, which almost covers the whole optical communication C-band window. The presented device shows a compact size within 40-35 μm and a high output efficiency, and has potential application values to photonic integrated circuits.