Fatigue changes the mechanical behavior of reinforcing bars, significantly reducing the performance of aging concrete structures. This study performed a fatigue loading test on post-tensioned prestressed concrete (PC) beams with various corrosion levels of prestressed tendon. The effect of corrosion on fatigue life and failure modes of test beams were discussed. Steel bars at different distances from the fracture location in beams were removed for static tensile tests. The mechanical properties of the steel bars after fatigue in different locations relative to the failure point were analyzed. A quantitatively constitutive relationship was proposed to characterize the mechanical behavior of steel bars after fatigue. This model can consider the effects of fatigue stress range, fatigue cycles, and fatigue fracture location of steel bars. The experimental results showed that the failure of corroded post-tensioned PC beams initiated from the brittle fracture of prestressed tendon, promoting the fatigue fracture of steel bars. The proposed models were verified by the test data in the literature. The theoretical values were in good agreement with the experimental results. The proposed constitutive relation model considering the location effect can be used to model the uncertain effect caused by fatigue.