In the welding process,it is very important for welding torch automatically tracking the weld seam to assure welding quality and improve welding automation level.The striking point of seam tracking technique is how to sense the seam position.Through deep going research on activity of arc light emitting and linking with the practical welding condition,a image sensor system was built in this paper,which could detect the weld seam position utilizing the arc light.This system solved many difficulties in hardware,for example,the synchronization between image acquisition and welding current,the enhancement of image quality and so on.Moreover,the difficulties in computer software were solved as well such as matching of image quality with image threshold,detecting of weld location and so on.The result of using this system in practical welding production shows that the system could fully satisfy the requirement of automatic seam tracking to acquire high quality weld in many respects such as detection precision,processing speed,stability,reliability and adaptation to practical industry condition.