Abstract: Residual compressive plastic strain presents in the weld from traditional views, now some literates think that weld is formed during the cooling process which isn' t concerned with the heating process.Therefore, they concluded that not the compressive plastic strain but the tensile plastic strain presents in the weld, in order to indicate the mechanism of the formation of welding residual stress and strain, welding longitudinal stress and plastic strain of the low-carbon steel are simulated by finite element method.The simulated results show that residual compressive plastic strain and tensile stress are found in the weld longitudinal plastic strain decreasing from the weld centre to the edge of the base metal where plustic deformation won' t take place.