Abstract: The width-to-thickness ratio of plate elements, for an axially loaded box-shaped member, should not be greater than 40√235/f_y according to the Code for design of steel structures (GB 50017-2003). However, in practical engineering the width-to-thickness ratio is up to 120, much larger than that specified in code. The ultimate capacities of axially loaded welded square box section members made of high strength steel (nominal yield stress is 460 MPa), with width-to-thickness ratios of 35, 40, 45, 50, 55, and 60, were analyzed by the ANSYS program. In numerical modeling, the geometric imperfections (both the global and local initial deflections) and residual stresses, as well as the geometrical and material nonlinearities, were taken into account. In addition, the curve b provided in the Code (GB 50017-2003) was replaced by curve a. The direct strength method was applied in the calculation, and the calculation results were compared with the numerical results. Furthermore, a modified direct strength method was proposed for the members with large discrepancy. The investigations show that the finite element model presented in this paper can simulate the behavior of nonlinear buckling of axially loaded box-shaped members. For the members with width-to-thickness ratio greater than or equal to 45, good agreements are observed between the results calculated by the finite element model and direct strength method; however, for members with width-to-thickness ratio less than 45, the errors are significant. The proposed modified direct strength method is in good agreement with the experimental results.