1. A document image enhancing method comprising steps of: reducing noises in an original block image, resulting in a noise-reduced block image; segmenting the noise-reduced block image and obtaining position information of line images; pre-analyzing the line images and the inter-line images in the original block image, obtaining properties of the line images and the inter-line images; reducing noises in the line images in the original block image based upon said properties, resulting in noise-reduced line images; labeling the connected components in the noise-reduced line images and obtaining the densities of different connected components in the noise reduced lines images; based on the connected component densities, sorting the noise-reduced line images into three types including normal line image, broken-stroke line image and hollow-stroke line image; enhancing the noise-reduced line images based on their types and said properties, obtaining enhanced line images, which as a whole, along with the inter-line images obtained in the step of reducing noises in the original block image, constitutes an enhanced block image.