In order to solve the problem of precision fitting in numerical control machining of the skin seam, a method for extracting repair amount of the skin seam was proposed based on scan line point cloud. Firstly, the point cloud was processed in blocks and the boundary feature point of each scan line was extracted by the distance from the point to the fitting plane. Then, the feature points were denoised using chord height difference method and the middle point was selected as the initial point to sort on both sides. Finally, the skin seam docking model and laser scanning boundary were analyzed and the feature points were extended to the docking direction to compensate. Field experiments showed that this method could provide exact repair amount with an accuracy of 0.06mm, which could meet the needs of numerical control machining.