In order to study the flexural performance of the carbon fiber mesh-polymer mortar composite reinforced beam under the secondary loading effects，and improve the calculation theory of its ultimate flexural bearing capacity and deflection. Based on the constitutive relationship of the concrete in compression zone，the ultimate flexural bearing capacity formula of the carbon fiber reinforced beam under different failure modes was derived，and the calculation criterion of identifying the failure mode of the reinforced beam was proposed. According to the stiffness analysis method，the short-term stiffness calculation formula of the carbon fiber reinforced beam under the secondary force was derived. The method of interpolation fitting was used to process binary equations such as M-x_n、 M- β1，which was applied to the deflection control check calculation of the reinforced beam under different load levels，to avoid the solution errors caused by the parameter value problems. The bending test was carried out on 4 carbon fiber mesh reinforced beams under different preload amplitudes，and the theoretical calculation value was compared with the experimental value. The results show that the theoretical calculation value of the carbon fiber mesh reinforced beam is relatively consistent with the experimental value，and the relevant calculation method has a certain reference to guide the design of actual engineering.