The failure of the radial steel gate is mainly caused by the buckling instability of its supporting arm, and the stability of the arm is the premise of safe and stable operation of the whole gate. Aiming at the current problem of insufficient calculation accuracy and efficiency of the arm stability, it is of great significance to find an equivalent simplified model which can be used to calculate the actual stability bearing capacity of the arm, and determine the comprehensive initial imperfection amplitude considering the influence of residual stress on the stability. Taking the welded I-shaped arm with two hinged ends as the research object, we investigated the interactions between the stability bearing capacity and the factors of residual stress distribution mode, residual compressive stress peak value and geometric initial imperfection amplitude using a nonlinear buckling analysis method. Then the calculation results were compared with the specified value in the Standard for Design of Steel Structures(GB 50017—2017). The results show that for large flexible arm, the comprehensive initial imperfection amplitude is L/600, whereas for medium and small flexible arms, it is L/400. The proposed equivalent model fully considers the influence of cross-section residual stress and geometric initial imperfection on the stability bearing capacity of the arm. Its calculation results are in good agreement with the specified value in the standard, so it can replace the original complex nonlinear stability model for gate arms. The research results can provide some reference for the application of the model in actual projects.