Bonding degradation at interface is one of main damage forms of composites, especially under fatigue loading. Interfacial bonding degradation of FRC under two-stage tension loading is studied, which is base for variable-amplitude cyclic loading existing widely in actual engineering. Based on the shear-lag model and considered the asymmetry of interfacial damage, the mechanical governing equations of fiber and matrix are established and related solutions are obtained firstly. Two kinds of loading models are chosen, one is low-high alternate loading, and the other is low early and high late loading. By the aid of the Paris law and the energy release theory, a relationship between debond rate and cycle number is established. Then the interfacial debonding is simulated under the two-stage tension loading. The rules of the crack growth are analyzed for low-high two-stage loadings. It is found that stress amplitude has great influence on interfacial debonding under two-stage loading. Low stress amplitude in a certain range can postpone interfacial bonding degradation. And interfacial damage extent is greater than that under constant-amplitude fatigue loading. Present study is helpful for analyzing the fatigue damage of engineering materials and structures.