1. A grading method for digital image quality, comprising: obtaining, by a processor, a digital image; extracting, by the processor, n main body region blocks from the digital image, wherein n is a natural number; calculating, by the processor, a first ratio value of an area of a plurality of main body region blocks to a total area of the digital image, a second ratio value of an area of background region blocks to the total area of the digital image, and a normalized distance value from a plurality of pixel points in the plurality of main body region blocks to a center pixel point of the digital image, wherein the background region blocks are residual region blocks after the n main body region blocks are extracted from the digital image; calculating, by the processor, a quality score value of the digital image through a preset digital image quality score conversion relation based on the first ratio value, the second ratio value, and the normalized distance value, wherein the preset digital image quality score conversion relation is a directly proportional function of a sum of the first ratio values of the plurality of main body region blocks, a sum of the second ratio values, and a sum of the normalized distance values of the plurality of main body region blocks; and grading, by the processor, a quality of the digital image according to the quality score value of the digital image and a preset digital image quality score threshold value.