A multi-channel wavelength division multiplexing light emitting device includes a casing and an optical communication assembly accommodated in the casing. The optical communication assembly includes a substrate, a plurality of first light emitting units disposed on the substrate, a plurality of second light emitting units disposed on the substrate, a first wavelength division multiplexer, and a second wavelength division multiplexer. The first light emitting units are arranged to correspond with the first wavelength division multiplexer. The second light emitting units are arranged to correspond with the second wavelength division multiplexer.