Aiming at the low efficiency of measuring the size of a manual pinch with a vernier caliper, an improved adaptive Canny operator is proposed to measure the size of a manual pinch. Firstly, the mean shift method is introduced to segment the image, and the image color becomes gradual and the fine grain texture is gentle. Secondly, the average pixel value of approximate contour is selectively used to replace the high threshold of edge detection, to achieve the purpose of better adaptive edge extraction. Thirdly, Scharr operator replaces Sobel operator to enhance the ability of weak edge extraction. Finally, the small connected domain is removed, and the principal direction of the contour is obtained by principal component analysis ( PCA), and the straight-line equation of the principal direction is obtained. The length of the pinch is obtained by the maximum projection distance of the contour point on the straight line, and the width of the pinch is obtained by the maximum projection distance of the contour point in the vertical direction of the straight line. Experiments show that this method has significantly improved the recognition rate of irregular objects more than other methods, with an accuracy of 97% and an error of about 2 mm from the real value. Through the comparative analysis of strong light, weak light and sidelight experiments, this method is less affected by light and meets the needs of industrial automation.