Brake pad dimension detection is one of the important indices of brake pad quality. In order to quickly and comprehensively detect the dimension of the brake pad, this paper presents a comprehensive brake pad dimension detection method based on tolerance design principle and image processing. Firstly, according to the requirements of the design drawings, the dimension tolerance design principle is used to reversely design the brake pad allowable dimension tolerance control zone, which is taken as the comprehensive dimension test standard template. Secondly, the CCD camera is used to acquire the image of the brake pad to be tested, which is then filtered, processed with binarization; a binary image is obtained and the minimum circumscribed rectangle optimization algorithm is used to determine the scaling factor, and the actual image the same as the standard template in pixel scale is obtained. Finally, at the midpoint of the line connecting two cycle center coordinates of the actual image and standard template, two images with same size are cut out. Edge detection is performed on the actual image to get the actual contour image, then the actual contour image and the standard template are added up, and then from the difference of the numbers of the white pixels for the added image and standard template, the detection result is obtained．The experiment result shows that the detection accuracy can reach 1 pixel, the detection speed reaches 1 piece per 5 seconds, and the proposed method can detect weather all the dimensions of the brake pad are qualified.