The existing cable elongation at break measurement methods have defects of poor stability and low automation degree. To address these issues, a method based on the improved L-K optical flow is proposed. Firstly, black ring markers at both ends of the cable surface are used to extract feature points. Secondly, edge extraction and roundness calculation are utilized to obtain the initial centroid positions of the markers. The centroid with the feature points detected by the Harris algorithm is matched according to the principle of minimum Euclidean distance. In this way, the automatic selection of feature points is achieved. During the process of cable stretching, the improved L-K optical flow method is used to track the feature points and calculate the acceleration and spacing of the feature points in real time. An acceleration mutation criterion is proposed, which can accurately locate the frame where the cable breaks. The problem of poor accuracy of artificial judgment at the cable rupture moment is solved. Finally, according to the distance between the feature points before and after the cable deformation, the elongation at break of the cable is calculated. Experimental results show that the average error of cable elongation at break is 2. 78% , and the average speed of the algorithm is 21 frames/ s. Results indicate that the algorithm can measure the cable elongation at break quickly and accurately.