In this paper, a dynamic detection system for thermocouple cable insulation defects is proposed. The common thermocouple cable defects are insulation cracking and cable joints. These two defects can be identified by detecting the cable diameter. Detecting such defects through the human eye is time consuming and labor intensive and does not guarantee quality. The method in this paper is to make the thermocouple line pass the field of view of the line camera when moving at high speed. Then use the edge detection algorithm to calculate the cable diameter. After comparing with the standard cable diameter, analyze whether the cable is normal or there is a defect in the joint or the insulation layer. In this paper, the edge detection algorithm is used to realize the dynamic detection of thermocouple cable defects. Improve the reliability of the products, improve the mechanization of the production process, and in a result of labor saving.