In the related equipment of the third generation nuclear power unit,the CVS mixed bed and cation bed are high pressure vessels made of austenitic stainless steel. The wall thickness of the cylinder reaches 122 mm,and there is no manhole design. The circumferential weld between the head and the cylinder can only be welded from one side,which puts forward higher requirements for the welding process and welding quality control. Any weld defect is difficult to repair,and it has great influence on weld quality. In this paper,the control measures of welding process,welding quality control and welding shrinkage are established. The final weld quality of the product meets the design requirements,which provides reference for the welding of similar thick wall stainless steel vessels.