There are two pairs of double-layer thick forged plates welding on the shell as horizontal supports in a certain third-generation steam generator. The welding seam length is 1 900 mm,and the welding seam thickness is 130 mm. After welding,the roundness of the shell should be less than 5 mm,and the parallelism of the upper and lower surface of the horizontal support should be less than 1.5 mm. It is difficult to control the deformation because of the high welding stress. Through the analysis of the welding deformation of the horizontal support,a set of welding groove with narrow gap is designed. The rational welding sequence of the alternate welding is set. We use narrow gap submerged arc welding method. The deformation control measures such as anti-deformation tooling is used to effectively control the welding deformation and ensure the parallelism of horizontal support and roundness of the shell after welding of the horizontal support. According to the analysis of the actual deformation data,the structure of the horizontal support is optimized,and the manufacturing cost of the horizontal support is reduced. The relevant measures in this paper can provide reference for similar thick plate welding deformation control in the future.