Embodiments of the present application provide a method and an apparatus for transmitting an optical signal and a wavelength selective switch. The transmission method includes: performing diffraction processing on an input optical signal, to obtain signal light and crosstalk light. The signal light is output to a target output port in a plurality of output ports. The diffraction processing includes deflecting, in a second direction, a diffraction direction of a part or all of the crosstalk light, so that the part or all of the crosstalk light is output to an area outside the output ports.