A method for detecting a facial expression and repairing a smile face of a portrait photo includes the steps of: detecting a location and a range of a mouth region in an inputted portrait photo; capturing a patch in the mouth region and a predetermined peripheral range thereof; executing a comparison process to a smile state or a stiff state of the mouth region in the patch by a mouth state classifier; executing a calculation process to a repaired region of the mouth region when the mouth region is determined to be in the stiff state, in order to calculate a location of a plurality of feature points in the repaired region of the mouth region; and executing an image warping process to the location of the feature points and adjacent pixels thereof, for generating a portrait photo showing a smile state.