Abstract: An eigenline in the electrofusion joint was observed by using the phased array ultrasonic focusing technique. A new method called “eigenline method” was developed to assess cold welding with an empirical formula by measuring the distance between the line and metal wires. Further research showed that the developed method could be used to assess the welding technology in ultrasonic test realtime video by measuring the moving speed of the eigenline and the distance between the line and metal wires. Based on ultrasonic test realtime video experiments and the developed theoretical model of welding process for electrofusion joints, the optimized welding technology for the joint was obtained, which greatly reduced the time and cost in development of electrofusion joints.