Abstract: In current bridge reinforcement codes, there is no formula for calculating the stability of axially compressed steel-bridge member strengthened under load. In order to explore the influence of three factors, i.e., initial load, strength and length of reinforcement, on the stability of reinforced member, 11 specimens were designed and tested. By analyzing the stress distribution at limit state, the initial load influence is explained. Through the study of the calculation method in current specifications and the comparison with the test result, the limitations of each formula applied to the calculation of axially compressed steel-bridge member strengthened under load are analyzed. Through the analysis of 'collapse theory', an improved formula is proposed. Through experiment, calculation and theoretical research of the reinforcement scheme in this paper, some factors affecting the stability of axially compressed member strengthened under load are clarified, and the improved formula is put forward, which can provide reference for the similar projects and steel bridge reinforcement specifications in the future.