Abstract: The fatigue strain evolution equation was obtained by fitting the experimental data of the maximum fatigue strain and the fatigue residual strain, and the correlation coefficients were all above 0.97. It was observed that the fatigue modulus was inversely related to the fatigue strain under constant amplitude fatigue loading. Thusly the fatigue modulus evolution equation was obtained according to the fatigue strain evolution equation by use of symmetry. It was found that the fitting curves expressed by the fatigue modulus evolution equation coincided with the experimental curves very well, and the correlation coefficients were all above 0.99. Finally, the damage variable evolution curve was obtained by use of the fatigue strain and fatigue modulus defining the damage variable. It was also found that the damage variable evolution curves defined by the maximum fatigue strain and fatigue residual strain were basically consistent and the deviations were quite small, but the damage variable evolution curve defined by the fatigue modulus was significantly larger than that defined by the fatigue strain.