The invention relates to the technical field of splicing welding of composite pipes of overlong waste heat boiler in the coal chemical industry, in particular to a welding structure for realizing constant-strength splicing of the metal composite pipes and a welding method of the welding structure. The welding structure comprises a composite pipe base layer and a composite pipe composite layer, and a groove structure of the composite pipe composite layer can ensure one-side welding with back formation and evenness between weld root reinforcement and a base material. Compared with the prior art, the welding method has the advantages that the composite pipe composite layer is made of a welding material corresponding to the composite pipe composite layer, and the composite pipe base layer is made of a welding material corresponding to the composite pipe base layer, so that constant-strength splicing of the composite pipes can be realized; besides, a welded joint eliminates adverse factors such as metallographic structure change and a carbon migration diffusion layer in a fusion zone, which are caused by welding of dissimilar materials, failure of dissimilar steel welded joints due to severe working conditions of high temperature and the like can be avoided, and the operation cycle of a waste boiler is effectively prolonged.