Aiming at the serious safety accidents caused by the defects of positive and negative electrodes of lithium batteries, a defect detection method for the distance between positive and negative electrodes of lithium batteries is proposed. Firstly, the X-ray image of the lithium battery is obtained, the region of interest in the image is intercepted by the watershed algorithm, and the region of interest is rotated and corrected. Secondly, aiming at the problem that it is difficult to segment the negative linear region of the lithium battery, a horizontal gradient template is designed to extract the positive boundary and intercept the negative linear region of the lithium battery. Then, a multi-scale retinal enhancement algorithm and an extended differential template are used to extract the negative line. Finally, the vertical coordinates of the line are obtained by horizontal projection of the extracted negative line, and the coordinates of the positive and negative pole points are obtained according to the positive gradient and the extracted negative line, so as to obtain the positive and negative distances of the lithium battery. The experimental results show that the proposed algorithm has low missing rate and stable operation and meets the industrial requirements.