In a method of audio signal processing, a hearing aid system (2) is configured to: collect sound information of a surrounding environment of the hearing aid system (2); capture an image (40) of the surrounding environment of the hearing aid system (2); perform a directional signal processing operation on the sound information so as to generate an output audio signal; the output audio signal containing an extracted voice signal that comes from an area corresponding to a location of a target object in the image (40); and output the output audio signal.