Abstract: An automatic arc welding machine for saddle joint seam on large diameter cylinders was designed and its structure,components,characteristic and working principle were discribed in this paper.Author deduced the equations for calculating the geometry and dimensions of the cams which were the key parts to control the moving locus of gas shielded arc welding torchs: l=l'-Z'=l'-(R²-r² cos² ψ)^1/2-rsinψcosα/sinα The welding machine was successfully applied to the automatic welding production of boiler cylinders.The production practice had shown that this automatic welding machine was suitable for the saddle joint seam and good results were achieved.It was a new type of low cost automatic welding equipment.