Abstract: The method of rectifying the roundness of welded thin-walled cylinders was proposed by extruding the metal in and around the weld. The effect of roundness rectification by extrusion and the mechanism in which the effect lies were investigated by finite element method (FEM). The cylinder welded longitudinally has a peach shape in radial direction and significantly distorts at two ends. The simulation results show that the extrusion method can effectively improve the roundness of the welded cylinder. The effectiveness of rectifying the roundness of welded thin-walled cylinders by extrusion was verified with self-made devices. The experimental results show that the roundness at the ends of welded stainless steel cylinders with 2 mm in wall thickness, 300 mm in length and 280 mm in external diameter can be reduced to about 0.5 mm by extruding the metal in and around the weld.