This paper deals with precautions needed for an accurate numerical analysis of welding residual stress and deformation. By comparison of numerical results with experimental data, the precautions of the thermal-elastic-plastic FEM are investigated. The butt joint made by one pass and one layer welding was selected as the object of study, because the feature of butt joint is to be made one jointed plate from two separated plates by the welding. Then the boundary conditions change by the moving of the deposited metal melted by heat souce. The obtained precautions are as follows: The element division is determined by the size of melting point of mother material on the welding direction and the size of inherent strain area calculated by the fix-bar model on perpendicular to the welding direction. The boundary condition of butt joint is expressed by the method that the perpendicular restraint to the welding direction along weld center line does not exit before the melting metal being not deposited and exits after being deposited.