Abstract: Addressing the problem concerning weakening of weld seam mechanical properties of the spiral weld pipe，analyses of the welding pool-to-weld bead forming process and the forming stress are conducted with the metal welding and material mechanics theories.As a result，the fact as below is identified.Owing to the unavoidable existence of small quantity of harmful chemical elements in the weld，during the process where the welding pool is transforming from the liquid-solid phase into the solid-liquid phase，the brittle temperature zone is developed，and when the residual stress as transmitted into this zone，the intermolecular distance is increased，and this causes forming of the granular space，which results in weakening of the weld mechanical properties.Accordingly，it is possible to improve mechanical properties of the weld by means of using welding material containing less harmful chemical elements，and reducing the forming stress.