The invention discloses a welding process used for horizontal support connecting plates (1) and a cylinder body (2) of a steam generator. Single-side narrow-gap grooves are adopted; an arc-shaped flame heating device is adopted for heating; a welding gun (3) is used for welding. By automatic submerged arc welding, the grooves in two sides of each group of the horizontal support connecting plates (1) are alternately welded, and subsequent welding is carried out after magnetic powder testing is qualified. According to the welding process provided by the invention, automatic submerged arc welding in narrow gaps is realized, so that the welding process is stable, the weld formation is uniform, the side wall fusion is good, the welding deformation is effectively controlled, and various nondestructive testing results all meet product design requirements while the welding efficiency is remarkably improved.