An apparatus (300) and a method for processing an optical signal are provided. The apparatus includes: an optical beam splitter (310), configured to perform beam splitting processing on the optical signal to obtain a plurality of optical components; an optical delay unit (320), configured to perform delay processing on the plurality of optical components obtained by the optical beam splitter (310) by using beam splitting processing; a mode division multiplexer (330), configured to perform mode conversion on the plurality of optical components obtained by the optical delay unit (320) by using delay processing, to obtain a plurality of optical components, where any two of the plurality of optical components obtained by using mode conversion are corresponding to different guided wave modes, and the mode division multiplexer (330) is further configured to superimpose the plurality of optical components obtained by using mode conversion, to obtain a superimposed optical signal; and a photodiode detector (340), configured to convert the superimposed optical signal obtained by the mode division multiplexer (330) by using superimposition into a current signal. According to the apparatus (300), a plurality of optical components can be superimposed in an optical domain, and a quantity of photodiode detectors (340) can be reduced.