The bond decay at the bar–concrete interface under variable fatigue loads is investigated in this paper. Two kinds of loading paths are considered: low- and high-amplitude cycles. Based on the shear-lag model, the governing equations of the problem are established and solved with reference to three different interfacial degradation models. By the aid of Paris formula, the interfacial debonding rate, the debonded length, and the pull-out force are studied. The effect of the amplitude of the middle applied load and of several material parameters on bond decay is discussed. It is found that the power-degradation model is suitable for short embedment lengths, while both the linear-degradation model and the modified power-degradation model are advantageous in study of bond in long embedment length. It is also found that interfacial friction plays an important role in resisting interfacial debonding, in spite of the effects of Poisson's contraction.