1. A grading method for digital image quality, comprising: obtaining, by a processor, a digital image; extracting, by the processor, a plurality of main body region blocks from the digital image, a number of the plurality of main body region blocks comprising a natural number; calculating, by the processor for each of the main body region blocks, a first ratio value of an area of a respective 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 respective main body region blocks to a center pixel point of the digital image, the background region blocks comprising residual region blocks after each of the 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 values, the second ratio value, and the normalized distance values, the preset digital image quality score conversion relation comprising a sum of the first ratio values the second ratio values, and the normalized distance values; 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.