1. A method for driving virtual facial expressions by automatically detecting facial expressions of a face image, which is applied to a digital image capturing device, comprising: a face detection block, for automatically detecting positions of a face, eyes, a nose, and a mouth in a target image captured by said digital image capturing device, and converting said detected positions into a standard image for a virtual front face, and calculating initial positions of all related key points of eyes, nose, and mouth of said target image on said standard image; a key point detection block, for calculating exact positions of said key points of said target image on said standard image, and inversely aligning said exact positions of all key points of said standard image onto said target image, and labeling said exact positions as exact positions of said key points on said target image; and a key point tracking block, for automatically tracking the positions of all key points of other target image captured by said digital image capturing device later, and correcting said exact position of each key point on said standard image.