Using cymbiform submerged arc welding; when welding, workpiece is fixed, but portal frame moves. Gantry frame adopts frequency timing; equipped a set of welding torches at each 2 groups of guiding column on the beam; use electric dynamo driving to adjust shifting up and down; use manual adjusting device to adjust shifting left and right. Lead arc device do gmechanic tracking to welding seam, which can get high quality of welding line; automatically recycle the flux and save labor.