Abstract: In the welding procedure of the nozzle-to-shell joints on pressure vessel,instead of the traditional manual arc process an automatic submerged-arc welding process has been developed and applied in order to tighten the welding control and attain a new heights of the welding efficiency.For this purpose an automatic welding machine(Mode MZM-500) with a non-contact type high precision head following device to the saddle-shaped curve was made.Welding performance tests and welding production of nozzle-to-shell joints on a large pressure vessel for a power station boiler were carried out and an excellent result was obtained.As compared with the manual arc process,the present submerged-arc welding process may raise the productivity up to 3 times and reduce the fabrication cost,the preheating temperature has been lowered,and the working condition of the welders has also been gotten better;at the same time,it gives a high quality assurance in the welded nozzle-to-shell joints.