The invention relates to a method for iron core radial tightening of a steam-turbine generator. According to the method, an iron core to be tightened is supported by multiple sets of clamping ring cages, each set of clamping ring cage comprises two half rings, a welding groove is formed in the butt joint portion of every two half rings, and after the iron core is tightened, every two half rings of the sets of clamping ring cages are welded one by one. According to the method for controlling axial gaps between the clamping rings of the iron core, materials are saved, vibration is reduced, noise and friction corrosion are reduced, operation is simple and convenient, and the safety is good.