Abstract: Studying the bearing capaicty characteristics of in-built concrete flexural members and establishing the capacity calculation method,can provide a reliable theoretical basis to the analysis of the bearing capacity of in-built reinforced concrete structure,and has important theoretical significance and value of engineering application for the security analysis and control of reinforced concrete construction and the process life cycle analysis of reinforced concrete structures.Two concrete ages of 14,180 d were selected,the bearing capacity of four reinforced concrete beams were tested,failure modes of specimens and deformation characteristics were observed,the difference of ultimate moment was compared,the change laws of some variables such as deflection,strain distribution across the concrete,free end slip of reinforcement were analyzed.Through numerical simulation,the main force characteristics of in-built reinforced concrete flexural members were analyzed.Combining with the limit equilibrium theory,the capacity calculation model of in-built RC flexural member was established,the bearing capacity time-varying law of reinforced concrete flexural strength during construction was analyzed.