Abstract: The serious residual stress and distortion are the weld defects associated with welding thin plate structure of high strength aluminum alloys.By applying a longitudinal tensile stress less than yield strength of the material,the residual stress of the flat butt welding was analyzed by the ellipsoidal heat source model.The results show that it can improve the residual stress distribution of welded joint and the peak value of the residual stress reduced evidently.The greater the pre-stress,the lower the residual stress.So welding distortion reduced.The values of deflection between conventional welding and welding with pre-stress were compared.It is found that the deflection reduced with the increase of longitudinal pre-stress.The maximum deflection of thin plate decreased from 38 mm under the condition of conventional welding to 18 mm and 7 mm respectively when 0.5σs and 0.7σs were applied.The value decreased to 17% of that in conventional welding.The reliability of numerical simulation results is proved by the experimental results.