In order to predict scientifically and extend properly the useful service life of aged existing reinforced concrete bridges, considering the mass loss ratio and cyclical stress amplitude, axial tensile fatigue tests were conducted on 25 naturally corroded reinforcing steel bars in air.The fatigue curves of slight, medium and extensive corrosion steel bars were obtained by statistical regression analysis on the basis of taking into account the influence of corrosion on the fatigue properties.The deterioration rules in fatigue life of the slight, medium and extensive natural corrosion steel bars were investigated. Then, suggested values of the allowable stress amplitude corresponding to different target fatigue lives under the 50% guarantee rate were proposed for slight, medium and extensive steel bars naturally corroded，and compared with accelerated corrosion steel bars with the same level of corrosion. Test results indicate that the relationship between stress amplitude and fatigue life on a logarithmic scale for the slight, medium and extensive steel bars naturally corroded shows linear relation.However, a significant reduction in fatigue life was observed, especially under higher stress amplitudes.