Abstract: Numerical simulation method is employed to discuss welding longitudinal plastic strain distribution.Residual compressive plastic strain presents in the weld from traditional views, but the new viewpoint supports that the tensile plastic strain presents in the weld.Based on the two different viewpoints, the welding longitudinal plastic strain distribution is analy sed with and without the fusion phenomenon according to the calculation results.The simulated results show that there exists little different in the two situations, and compressive plastic strain in the heating process is larger than tensile plastic strain in the cooling process.When a fusion phenomenon is considered, the heat affected zone still keeps the compressive plastic strain all the time, and only the plastic strain value in the cooling process is smaller than the one in the heating process.