The invention belongs to the field of digital image processing and particularly provides a low-rank constraint-based license plate correction algorithm for a positioned license plate by means of a symmetry characteristic of the straightened license plate by converting the symmetry characteristic into a low-rank characteristic of an image matrix. According to the algorithm, by using a convex optimization iterative algorithm, the minimum rank of the image matrix is obtained through continuous change of a rotary parameter, wherein the rotary parameter is the final result; and a ticket is rotated to a proper position according to the rotary parameter. The algorithm provided by the invention can be used for accurately correcting a deformed license plate to a proper position so as to provide guarantee for identifying the license plate.