To study the deformation performance of CFRP-strengthened corroded RC (reinforced concrete) beams under fatigue loading, a numerical analysis method based on the fatigue damage failure process was used, and the fatigue constitutive models of each material were used as the basis. The finite element software ABAQUS was used to model and analyze the CFRP-strengthened corroded RC beams, and the degradation of bond performance between rusted reinforcement and concrete was considered in the simulation. Then, the flexural stiffness calculation method of CFRP-strengthened corroded RC beams was analyzed. Based on the validation of the finite element model, a regression analysis was performed on the mid-span deflection data of the simulated beam to establish a formula for calculating the mid-span deflection of CFRP-strengthened corroded RC beams under fatigue loading. Finally, the collected experimental data were compared with the calculation results using the mid-span deflection formula, which was validated to confirm its accuracy and reliability. The research results show that the mean, standard deviation and coefficient of variation of the ratio of experimental to calculated values were 0.95, 0.09 and 0.10, respectively, indicating that the calculated values of the proposed mid-span deflection formulation are in good agreement with the experimental values, which proves the correctness and high accuracy of the proposed formula for calculating the mid-span deflection. The formula can provide a theoretical basis for the evaluation of the deformation performance of CFRP-strengthened corroded RC beams under fatigue loading.