Bridges are the key components of civil infrastructure. Since decades ago, bridge strengthening and rehabilitation have become popular and their wider applications would be needed in the future. Although much research was carried in this field, some important problems including bearing capacity of reinforced beams didn’t form concordant views in this community. To research influence on bearing capacity of reinforced RC beams, we analyzed the loading process of RC reinforced beams from the beam-loaded-only stage to that of co-working of compound beams (including original beams and their strengthening layers), developed non-linear FEM program to simulate that loading progress, and finally validated the results with data from reference. This paper presents the theoretical analysis, and compares data yielded by program with those of experiment. We found that for adequate reinforcement beam the influence of secondary loading on the ultimate bearing capacity of reinforced beams is very limited. Our finding was confirmed by the experiment and FEM program.