Abstract: The mechanical properties and methods for calculating the bearing capacity of axially loaded steel columns reinforced by encasing them in reinforced concrete while already under load were researched through the integration of an experimental study, finite element simulation, and theoretical analysis in this paper. Through experimental study and finite element simulation analysis, the mechanical performance of axially loaded steel structure members reinforced by encasing in reinforced concrete while under load was studied. The influence of the initial loads, concrete strength, and steel strength on the ultimate bearing capacity of reinforced members was researched. The results showed that the bearing capacity and the rigidity of the steel members could be significantly improved using this method of reinforcement. The ultimate bearing capacity of reinforced members decreased with the increase of the initial loads, and increased with the increase of the steel strength. The concrete strength was the most important factor that affected the ultimate bearing capacity of reinforced members. The ultimate bearing capacity of reinforced members increased significantly with the increase of the concrete strength. From analysis of the experimental data and the finite element analysis results, the quantitative results of the influences of concrete strength on the ultimate bearing capacity of reinforced members were obtained. Meanwhile, based on the analysis of the stress and strain state of the various parts of the reinforced members at the time of reinforcement and the time of failure, the bearing capacity calculation formulas were derived for axially loaded steel columns reinforced by outsourcing reinforced concrete while under load, and the results thereof are in good agreement with the test results.