A method of audio signal processing that is implemented by a hearing aid system, the hearing aid system including an image capturing module, a sound pickup module, and a processor, said method including the steps of: a) collecting, using the sound pickup module, sound information of a surrounding environment of the hearing aid system; b) capturing, using the image capturing module, an image of the surrounding environment of the hearing aid system; c) using the processor to perform a directional signal processing operation on the sound information collected by the sound pickup module so as to generate an output audio signal, the directional signal processing operation retaining sound that comes from areas corresponding to locations of a target object in the image and filtering out sound that comes from other directions, the output audio signal containing an extracted voice signal that comes from an area corresponding to a location of the target object in the image captured by the image capturing module; and d) outputting the output audio signal; wherein step c) includes the following sub-steps of: c1) identifying, using the processor, presence of human face objects in the image captured by the image capturing module; c2) determining, using the processor, object information corresponding respectively to the identified human face objects; c3) determining, using the processor, a likelihood classification for each of the identified human face objects based on the object information corresponding thereto, wherein, for each of the identified human face objects, the likelihood classification is related to a likelihood of a person corresponding to the identified human face object being a target speaker; and c4) selecting, using the processor, one of the identified human face objects as the target object, the selected one of the identified human face objects having the likelihood classification that indicates a relatively greater likelihood of the person corresponding thereto is a target speaker compared to other ones of the identified human face objects in the image; wherein in sub-step c2), the object information includes relative depth information and relative orientation information for the corresponding identified human face object relative to the hearing aid system, different relative depths being pre-assigned with different weights, and different relative orientations being pre-assigned with different weights, the likelihood classification for each of the identified human face objects being determined with reference to the weights corresponding to the relative depth information and the relative orientation information of the respective object information, and the relative orientation information indicating an included angle formed between a reference axis in the image and the location of the corresponding identified human face object in the image relative to the hearing aid system, the reference axis representing an axis of the image capturing module. a) collecting, using the sound pickup module, sound information of a surrounding environment of the hearing aid system; b) capturing, using the image capturing module, an image of the surrounding environment of the hearing aid system; c) using the processor to perform a directional signal processing operation on the sound information collected by the sound pickup module so as to generate an output audio signal, the directional signal processing operation retaining sound that comes from areas corresponding to locations of a target object in the image and filtering out sound that comes from other directions, the output audio signal containing an extracted voice signal that comes from an area corresponding to a location of the target object in the image captured by the image capturing module; and d) outputting the output audio signal; c1) identifying, using the processor, presence of human face objects in the image captured by the image capturing module; c2) determining, using the processor, object information corresponding respectively to the identified human face objects; c3) determining, using the processor, a likelihood classification for each of the identified human face objects based on the object information corresponding thereto, wherein, for each of the identified human face objects, the likelihood classification is related to a likelihood of a person corresponding to the identified human face object being a target speaker; and c4) selecting, using the processor, one of the identified human face objects as the target object, the selected one of the identified human face objects having the likelihood classification that indicates a relatively greater likelihood of the person corresponding thereto is a target speaker compared to other ones of the identified human face objects in the image;