Fiber-reinforced polymer composites have been increasingly applied as reinforcing and load-bearing components in building constructions and civil infrastructures. Long-term exposure to changing environmental and loading conditions leads to composite degradation, which is highly related to degraded structure and properties of fiber/matrix interface and consequent interfacial debonding between fiber and matrix at nanoscale. By simulating interfacial structure and interactions with atomistic precision, molecular simulation allows for a high fidelity to interfacial variations as affected by environmental and loading conditions. In this paper, molecular investigations of interfacial degradation between fiber and matrix under various environmental and loading conditions are reviewed. Model construction of interfaces formed by different fibers and matrixes and simulation of various environmental and loading conditions are firstly introduced. Afterwards, mechanical and adhesion properties of molecular interfaces obtained from deformation simulations are presented. Meanwhile, interfacial degradations under various environmental and loading conditions are discussed and underlying mechanisms are revealed. Further discussions on modeling and simulation of molecular interface are proposed for future investigations. Overall, this work reviews previous molecular investigations of interfacial degradation of composites under different environmental and loading conditions, which contributes to evaluation of interfacial behaviors of composite materials during long-term service life.