Abstract: In the root pass of pipeline MIG welding, the unpredicted gap variation of weld seam caused by groove-preparing or welding deformation has extremely significant influence on the quality of weld seam.For this reason, a weld seam formation control system based on an embedded DSP(digital signal processor) visual servo platform was established.In the visual servo platform, TMS320DM642 has been used to grab and process the image in the weld pool area and TMS320F2812 has been used to accomplish real-time adjusting of welding torch posture and welding parameters.With a particularly designed image processing algorithm, the position information of weld pool, the weld seam and feeding wire were detached and calculated successfully.Then, according to the visual sensing results of weld seam width, real-time control of weld seam formation of root pass of pipeline MIG welding was fulfilled through the online regulation of welding parameters.X-ray test qualified weld seams with good apperearance are obtained.