Abstract: The welding longitudinal plastic strain field was simulated by elastic-plastic FEM analysis software. The simulation results showed that,for butt weldment made of 2A12-T4 aluminum alloy thin plates with 200 mm in length,100 mm in width and 2 mm in thickness,only longitudinal tensile plastic strain exists in the weld metal after welding,while in the zone close to the weld,not only longitudinal compressive plastic strain but also longitudinal tensile plastic strain exist. The distribution of residual longitudinal plastic strain along the welding seam is not uniform,especially complex at the zone near the ends of weld seam. The uneven distribution of longitudinal plastic strain in the zone close to the weld can be attributed to the combination of the variation of temperature field with the difference of load-carrying state of the metal beside the heating source during welding.