Abstract: Rebar corrosion is the principal factor affecting the service performance of reinforced concrete (RC) structures. Corrosion reduces the effective area of rebars as well as performance, and weakens the pin bolt effect of rebar on concrete. In addition, when the rebar is severely rusted, the concrete cover breaks, and the bond behavior between reinforcement and concrete deteriorates, affecting the mechanical properties of RC structures. In this study, a three-dimensional numerical model for shear analysis incorporating the nonuniform corrosion of reinforcement was established using an RC beam as the research object. The effects of corrosion on the mechanical behavior of the RC beam were explored via a multistage analysis method (namely, corrosion-induced expansion stage and structural deterioration stage). To model and simulate the expansion of the corrosion products, nonuniform radial displacement was applied to the concrete surrounding the rebar. The cracking process and the damage patterns of concrete resulting from corrosion were obtained. Then, taking the corrosion state as the initial condition, the static load was applied to analyze the mechanical behavior of the RC beam. After verifying the rationality of the multistage numerical model, the effect of the corrosion of tensile reinforcement and the shear-span ratio on the shear behavior of concrete beams without web reinforcement was analyzed. The modeling analysis results show that the corrosion of longitudinal reinforcement causes obvious longitudinal corrosion fractures in the concrete beam. Moreover, with the development of corrosion, the cracking area of the protective layer increases, reducing the shear capacity of the beam significantly. Furthermore, the shear-span ratio has a larger effect on the shear capacity of noncorroded beams than that of corroded beams. Finally, based on the simulation results, the calculation formulas of shear capacity in relevant design codes were discussed, and a methodology for predicting the shear capacity of RC beams without web reinforcement was proposed.