To understand the effect of seawater corrosion on the stability of steel beams, a global buckling bearing capacity test is conducted on steel beams corroded via an electrochemical method while considering the initial geometric imperfections. The equivalent remaining thickness of the corroded steel beam is calculated based on the yield load obtained from material property tests. Subsequently, this novel approach for calculating the corrosion thickness is utilized to establish a numerical model to simulate the global buckling bearing capacity of the steel beam. Employing this method enables a parameter analysis to be conducted. Furthermore, a new formula for the global buckling of the ultimate bearing capacity of steel beams is proposed based on the differential equilibrium equation of the microdeformation of steel members subjected to bending and torsion. The virtual work principle is employed to develop this formula, which not only simplifies the calculation process, but also assigns clear physical significance to each term.