1. A method for face recognition through facial expression normalization, comprising: fitting an input two-dimensional face image into a three-dimensional face model by using a three-dimensional face database; normalizing the three-dimensional face model into a neutral-expression three-dimensional face model by using a neutral-expression parameter learned from the three-dimensional face database; converting the neutral-expression three-dimensional face model into a neutral-expression two-dimensional face image; and recognizing the neutral-expression two-dimensional face image from a two-dimensional face database.