Abstract: 3D optical surface scanning method was used in this paper to investigate the buckling distortion of mild-steel plate during bead-on welding. The full field distortions were obtained through measuring the deformation of the plate before and after welding,respectively. Based on the numerical simulation on the elastic buckling distortion with inherent strain method,it was found that the concave-convex type distortion belonged to buckling distortion. The residual stresses were measured with hole-drilling method,and the results showed that the high longitudinal compressive stress existed in the zone away from the weld,which resulted in the buckling distortion of the plate. Combining with numerical simulation,the 3D optical surface scanning system could be used to accurately investigate the buckling mechanism of welded thin plate.