1. A method for enhancing edge of a halftone image, comprising steps of: forming an analysis window with a pixel as a central pixel; the analysis window comprising the central pixel and neighboring pixels; quantifying the pixels to get a halftone table; assigning a designated value to each pixel for generating a halftone result; comparing the halftone result with an edge enhancement table having a plurality of dotting commands and a plurality of dot tables; the dot tables correspond to one of the dotting commands; and executing a halftone table adjustment when one of the dot tables is conformed to the halftone result; the adjustment comprising steps of: obtaining correspondent dotting commands according to the conformed dot table; calculating a plurality of adjustment values for the pixels according to the dotting commands and an error diffusion process; and adjusting the halftone table according to the adjustment values.