Abstract: The computation precision of metal image area and girth was increased, and the algorithm for traditional method of computing image was improved. When the improved chain code method was used to compute the area and girth of images, the turning points of image edges and the small ramp edges were considered.Using this algorithm, the computation error of image area and girth is less than 2 percents. Compared with the traditional algorithm(computation error is more than 10 percents), the computation precision of the proposed algorithm was greatly increased. It will meet the requirement of practical engineering.