A halftone image generation method used in a system including an image input module and a halftoning processing module for generating a halftone image is disclosed. First, an original image is received by the image input module and a dot diffusion process is performed to the original image to generate the halftone image using a first class matrix with a first size and a corresponding first diffused weighting matrix with a first diffused area size, wherein the first class matrix indicates a processing order of the dot diffusion process and the first class matrix with the first size, the first diffused area size and the corresponding first diffused weighting matrix with the first diffused area size are optimized results determined in advance by the halftoning processing module according to class matrixes of different sizes and diffused areas of different sizes.