Steel structures need to be reinforced during service to improve structural safety due to structural ageing, functional changes, and increased loads. To explore the strengthening effect of new materials for reinforcing steel structures, steel section beams were strengthened by wrapping reinforced RPC under load conditions, and their flexural performance was investigated. The failure process of the beam was analysed, and the influence of the initial load, RPC strength, and steel ratio on the flexural performance of strengthened beams was studied through the bending test of the test beam under the secondary load. The results show that under the influence of different parameters, the flexural bearing capacity of reinforced section steel beams is increased by 1.68 ∼ 2.08 times compared with that of pure steel beams and it has better ductility. No interfacial bonding damage occurs between the section and RPC during the stressing process, and works well together. A finite element simulation was carried out to verify the test results. The finite element simulation results were in good agreement with the test results, and the error was less than 10 %. The test parameters were expanded with the finite element analysis method. The ultimate bearing capacity of the specimen after reinforcement decreases with increasing the initial load and increases with increasing the RPC strength, steel ratio and thickness of the RPC cover. Finally, a formula for calculating the flexural load capacity of steel beams after being strengthened with encased reinforced RPC was proposed by modifying the existing calculation method. The test and the calculated values are in good agreement, and the ratio of the test value to the calculated value ranged from 0.924 to 0.961. The test results verified the applicability of the proposed calculation methods.