This paper aims to reveal the degradation of the flexural load capacity of reinforced concrete (RC) beams reinforced with aluminum alloy (AA) under atmospheric conditions. For the same, first, time-varying models of the flexural load capacity of RC beams were developed before and after reinforcement with AA and steel plate based on the time-varying properties of the materials. Subsequently, upon collecting existing test data, the reliability of the models was verified. Finally, the parameters of AA-reinforced and steel plate-reinforced RC beams were analyzed to investigate the influence of factors such as concrete cover depth, cross-sectional area of reinforcing material, and cooperative working coefficient of reinforcing material and concrete (demonstrate the ability of reinforcement material and concrete to work together) on the remaining service life, rust initiation time, and cracking time. The results show that under atmospheric environment, the flexural time-varying degradation model for RC beam can be divided into three stages: pre-rusting, rusting-pre-cracking, and post-cracking, with differences in the dominant factors at each stage. In terms of long-term performance, AA reinforcement is significantly more efficient than steel plate reinforcement. This study provides some theoretical basis for the design of AA reinforcement in the atmospheric environment.