A huge number of bone swabs are unearthed from the Chang'an city site of the Han dynasty, and the automatic measurement method of bone swab size based on digital image technology can improve the work efficiency at such sites. An approach for measuring the size of a bone swab image based on an adaptive threshold Zernike moment in various places is proposed to address the issue of low measurement accuracy brought on by variable bone swab textures and low edge contrast. First, the Canny operator is used for pixel-level localization. Then, the effective edge of each sub-region is fixed symmetrically; the weighted gray value of each sub-region pixel point and center point is calculated with Euclidean distance as the proportion coefficient; and the judgment threshold of the Zernike moment extraction edge of each region is used to extract the subpixel edge of the bone swab. Finally, to remove the invalid texture, the edge contour discrimination condition is added, and the precise contour of the bone swab picture is obtained; the size of the bone swab is estimated using camera calibration, and the irregular contour is obtained using the minimum circumscribed rectangle algorithm. The experimental results demonstrate that the root mean square error of the length and width of the bone swab measured by this method decreases by 1.3 mm and 1.2 mm, the average relative error decreases by 3% and 6.3%, and the average absolute error decreases by 1.23 mm and 1.08 mm, respectively, compared with other methods. Thus, the method can effectively enhance the measurement accuracy of the size of the bone swab with complex edge profiles.