Abstract: The weld width, the thickness of base metal, the curve of seam track and the weld center line of the work piece to be welded are inconsistent in welding of thick plate with V groove, so a set of swing welding program was designed. In the off-line programming software of robot, the modeling of workstation, off-line programming and trajectory simulation were carried out, and the simulation results meet the test requirements. The test system was built with six axis robot and Fronius welding machine. By using the classical single variable control theory, the optimum welding parameters were determined. The low carbon steel plate of 18 mm in thickness was used as the base metal, multipass filling welding and cover welding in V groove were carried out. The test results show that there is good fusion on both sides of V groove, and it is free of weld collapse, interlayer fusion, undercut and gap between weld passes, and weld appearance is shaped like a fish scale.