Abstract: The longitudinal main girder of a radial steel gate is essentially a curved beam, but the straight beam theory is commonly used for calculation of normal stress at its cross section up to the present. This article presents a new calculation method using curved beam theory and corresponding flexural differential equations. By deriving a formula of characteristic coefficient for stress calculation, we have formulated a method for calculating the eccentricity of the girder of a radial steel gate with box section, analyzed the influence of curvature on the distribution of bending normal stress over the curved beam section, and compared the accuracies of the two theories. Application to case studies shows an error of 14.23% in the calculation results by straight beam theory relative to the FEM calculation, which is only 2.23% in the curved beam theory. Thus, the curved beam theory is much superior and its calculation results should be more reliable.