1. A method for calculating facial straightness, comprising: capturing an image; obtaining a facial area and locations of eyes of the image; dividing the facial area into a first sub-area and a second sub-area according to the locations of eyes; producing a first symmetric sub-area symmetrical to the first sub-area; coupling the first sub-area with the first symmetric sub-area to form a reference image; obtaining a first characteristic value of the facial area; obtaining a second characteristic value of the reference image; and calculating straightness of the facial area according to the first characteristic value and the second characteristic value.