1. A document image enhancing method comprising using a processor to perform the 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; obtaining properties of line images and inter-line images by pre-analyzing the line images and the inter-line images in the original block image; reducing noises in the line images in the original block image based upon said properties, resulting in noise-reduced line images; labeling connected components in the noise-reduced line images and obtaining densities of the connected components in the noise-reduced line images; based on the densities of the connected components, sorting the noise-reduced line images into three types including normal line image, broken-stroke line image and hollow-stroke line image; and enhancing the noise-reduced line images based on their types and said properties, whereby enhanced line images are obtained.