An image adjustment method comprises: generate a grayscale value of a target pixel by weighted averaging a plurality of input sub-pixel data of the target pixel; calculate an average of grayscale values of all pixels comprising the target pixel in a first window to generate a foreground value of the target pixel; calculate an average of grayscale values of all pixels comprising the target pixel in a second window to generate a background value of the target pixel; according to the foreground value of the target pixel and the background value of the target pixel, obtain a just-noticeable difference value in a look-up table as a grayscale difference value; generate an adjusted grayscale value of the target pixel according to the grayscale difference value; and generate a plurality of output sub-pixel data according to the plurality of input sub-pixel data of the target pixel and the adjusted grayscale value.