Embodiments described and shown provide a signal transmission method, a signal receiving method, a related device, and a system. The signal transmission method includes: generating a single-wavelength optical carrier; splitting the single-wavelength optical carrier into N subcarriers having a same wavelength; generating a spreading code corresponding to each of the subcarriers to obtain N spreading codes, where a bandwidth of each of the spreading codes is less than or equal to a preset threshold; deserializing a to-be-transmitted data signal into N sub-data signals; modulating the N subcarriers based on the N sub-data signals and the N spreading codes, to obtain N modulation signals; and combining the N modulation signals into one combined signal, and outputting the combined signal.