Abstract: The ANSYS software was used to carry out a three-dimensional simulation for the longitudinal residual stresses in a welded I-section, and the course of stuffing, melting, freezing of the metal in a welding seam were simulated successfully by the method of combining the birth-death element and the heat generation rate. The residual stresses were taken when the temperature cooled to 70℃，and the influence of the width to thickness ratio of flange on the distribution of welding residual stresses were investigated. The calculation results show that the peak value of the welding residual tensile stress in a section almost reach f_y , the welding residual compressive stress peaks in a wide flange and in a narrow flange is 0.4f_y and 0.3f_y, respectively. The residual stress distribution curve in a web is trapezoidal, and the peak value of compressive stress closing to a wide flange and to a narrow flange reaches 0.4f_y and 0.3f_y, respectively. With the decrease of width to thickness ratio of flange, the welding residual stress peaks of the section decrease gradually and the distribution width of tensile area has an increasing trend.