Abstract: The structured-light technology of traditional seam tracking makes front guiding error great if the curvature is varying. To solve the problem, a structured-light pre-scanning technology with double lines is produced.The seam is scanned before welding along the planned track of the robot.A laser line is added under the welding torch tip to indicate the front guiding error that will be recorded on time sequence and be eliminated when welding.A seam tracking system and its mathematical model are established.An image processing system is advanced, which the integrations of image processing technologies including median filtering, threshold transforming, thinness transforming and subsection beeline fitting locate the seam middle exactly.Tests show that the technology combined with the image processing system has the characteristics of strong anti-jamming, little error and fast processing speed, and it can meet the request of real time tracking.