Abstract: Welding deformations due to circular welds in the large cylindrical shell is studied by the model test and numerical analysis.The result shows that the inclination of the flange's plane is induced by the circular weld at the joint between the cylindrical shell and the flange.The effect of the circular welds between the small cylindrical shells and the large cylindrical shell on the flange's plane is not obvious, but it induces the partial concave of the flange's plane in the circumference.The inclination slop of the flange's plane, which is induced by the circumferential weld at the joint between the actual vacuum vessel and the flange, is analysed by FEM.FEM result coincides well with the value which is analyzed by the model test.