1. A string extraction method for an image based on a multiple stroke width patterns matching, wherein modules for processing said method comprises a set of stroke width patterns, a unit for computing the similarity between each pixel and each stroke width pattern, a unit for integrating the similarities between each pixel and all stroke width patterns, a unit for binarizing the integrated similarity between each pixel and all stroke width patterns, and a unit for clustering the pixels whose binarized integrated similarities are 1, said method comprises steps of: matching stroke width patterns listed by said set of stroke width patterns with a whole input image through said unit for computing the similarity between each pixel and each stroke width pattern for obtaining the similarities between various stroke width patterns in each pixel location; processing a function operation to the similarities between each pixel and all stroke width patterns through said unit for integrating the similarities between each pixel and all stroke width patterns for integrating and obtaining a numeric result, wherein said numeric result represents the width pattern integrated similarity of said pixel; integrating said stroke width patterns of each pixel through said unit for binarizing the integrated similarity between each pixel and all stroke width patterns so as to convert the relatively larger similarities into 1, convert the relatively smaller similarities into 0, wherein the similarities whose values are 1 represent that the pixels corresponding to thereof are located in a certain stroke of a certain character within said string; and clustering adjacent pixels of said stroke width patterns whose binarized integrated similarities are 1 to form a string area through said unit for clustering the pixels whose binarized integrated similarities are 1, and extracting an area of the original input image corresponding to said string area to be a string image.