According to the transport mechanism of chloride ion in concrete, the fatigue damage of concrete under alternating load was discussed. The chloride diffusion coefficient was related to the area of cracks. The effect of fatigue damage on crack area expansion value of concrete was quantitatively analyzed from the microscopic point. Based on Ficks second law, chloride ion transport model of damaged concrete under alternating load was established. The calculated results were compared with the test values of indoor experiment. The results show that the calculated results from the proposed model are in good agreement with the test values. The applied theory and the proposed hypothesis of chloride ion transport model for damaged concrete under alternating load are reasonable.