A 1  ×  4 optical demultiplexer (OD) based on the self-collimation (SC) effect in hole-type silicon photonic crystal (PC) was proposed, and its performance was numerically demonstrated. The 1  ×  4 OD consists of two cascaded Mach–Zehnder interferometers and an independent mirror. It has a total of four splitters and five mirrors. The simulation results indicate that by selecting the correct optical path length, the input SC beam with four evenly spaced frequencies can be demultiplexed into four output ports in the 1  ×  4 OD. If the center working wavelength is 1550 nm, the free spectral range of the 1  ×  4 OD is about 23 nm, which almost covers the whole optical communication C-band window. The size of the structure is on the order of 50  μm. Because of its simple structure, small size, and single material, it has potential application value in future photonic integrated circuits.