A tensile fatigue experiment was conducted on Q235 reinforcing steels with different stress levels and fatigue cycles. The steels were soaked in a saturated Ca(OH)₂ solution, and the chloride threshold level(CTL)for corrosion of reinforcing steels was detected by combining the open-circuit potential with electrochemical impedance spectroscopy. The changes of microstructure caused by the tensile fatigue were observed with a scanning electron microscope(SEM). The results show that the CTL of steels degreased with the increase of the stress level and fatigue cycles. The grains of the steels became finer and the amounts of grain boundaries and phase boundaries increased after tensile fatigue. This change became more significant with the increase of the stress level and fatigue cycles. The refinement of grains and increase of the amount of grain boundaries will cause short-circuit diffusion and reduce the CTL of steels.