Abstract: Weld seam tracking is an important aspect of welding quality control and GMAW(gas metal arc welding) is widely used in industries.An image of root gap in GMAW was captured using CCD camera and composite filter lens.An algorithm was designed based on the analysis of gray characteristics of the root gap.Root gap edges could be extracted by the algorithm.Thus the gap and the deviation between torch and root gap centerline could be obtained.This work has laid a solid foundation to realize the GMAW weld seam tracking.