Abstract: In view of problems of single determination way and high computational complexity of joint elongation of steel cord conveyor belt, a calculation method of joint elongation of steel cord conveyor belt based on X-ray image was proposed. Histogram equalization and mean filter are used to pretreat image for improving image contrast and effectively solving the problem of uneven grayscale image of the steel cord conveyor belt. Sobel operators are used for joint edge detection and dynamic threshold process to extract upper and lower joints. Slope of fitting straight line of the upper and lower joints is used to determine whether the joint of the image to be checked is the same with the reference joint, so as to judge whether the joint is elongated or not. Outermost length on the left and right ends of the joint and average length of the joint are calculated according to the end points and the fitting lines of the upper and lower joints. The experimental results show that the method is computationally fast, efficient and can calculate the joint elongation easily and accurately.