A method, an image processing device, and a system for generating a disparity map are proposed. The method includes the following steps. Images of a specific scene are captured by two image sensors to generate two original images. A shrinking process is performed on the two original images to generate two shrunk images. A shrunk disparity map is generated by using the two shrunk images. A magnifying process is performed on the shrunk disparity map to generate a magnified disparity map. Whether each magnified disparity value of the magnified disparity map is reliable is determined so as to accordingly generate a refined disparity map.