Abstract: In order to study the residual stress of the welding seam of small diameter thin-walled metal straight seam welded pipes, the critically refracted longitudinal wave method was utilized. The acoustic elastic coefficients of the welded pipes were calibrated by the static load tensile test. The results show that the residual stress increases with the increase of the stretching distance; the residual stress keep rising with the increase of the welded pipe diameter at the same stretching distance; the acoustic elastic coefficients of the welded pipes are independent of the diameter. Combined with the blind hole method, the results show that the critical refracted longitudinal wave is reliable to evaluate the residual stress.