Abstract: It is analyzed the limitation of traditional viewpoint which related the welding residual stresses only to the existence of plastic compressive deformation in weld and base metal.The authors believe that the method used in material mechanics called "section separating method" is not suitable for analysis of transverse residual stress in butt joint and the restrained contraction of weld during cooling plays a very important pole to residual stress forming procedure.Although the actions of the residual compressive strains and the residual contractive thermal strains are equivalent for causing the welding residual stress,it is useful distinguishing them to develop new measures relieving and adjusting the welding residual stress in practical joints.It is analyzed and modified to the application of inherent strain theory that should include mainly plastic tensile strains to welding residual stress.The very high residual stresses are measured by Mathar method from the experiment of depositing a soldering pass onto the mild steel plate under the condition of the temperature rising of the base metal lower than that needed for plastic compressive strains appearing,which verified the important role of restrained contracting of weld to the mechanism of welding residual stress.It is found that the strains measured have a close relation to the environment temperature for the great difference in linear expanding coefficient between the steel and the solder.