Abstract: The interfacial damage of fiber-reinforced composite under two-stage tension fatigue loading is studied. Based on the shear-lag model, the governing equations of the problem are established and the related solution is obtained. By the aid of the Paris law and the energy release theory, the relationship between debond rate and debond force is established. As an example, the interfacial debonding is simulated under the two-stage tension fatigue loading. The rules of the crack growth are analyzed for several kinds of two-stage loadings. The effect of Poisson's contraction on interfacial debonding is studied also. The present study is helpful for analyzing the damage of engineering materials and structures under fatigue loading.