Abstract: To address the degradation in the mechanical properties of glass fiber reinforced epoxy composites (GFRP) under hygrothermal environments, the surface protection effect of a transparent elastic coating was investigated. Unidirectional and hybrid lay-up GFRP were studied. Moisture absorption, tensile, and tension-tension fatigue tests were conducted at 70℃ and 85% RH. SEM, FTIR, and TGA were employed to characterize the changes in microstructure, chemical structure, and thermal stability induced by hygrothermal exposure. In addition, the mechanical responses of uncoated and coated specimens under the HB and FAA criteria were compared, and the protective effect of different coating thicknesses was evaluated. The results showed that the equilibrium moisture absorption of unidirectional GFRP was about 0.72%. As the moisture absorption increased, the tensile strength decreased from 1645 MPa to 1027 MPa, and the fatigue limit decreased from about 40% to 34%. Similar degradation trends were observed under both the HB and FAA criteria, while the coating improved the tensile and fatigue properties under hygrothermal conditions. For hybrid lay-up GFRP, the coating with a thickness of 0.2 mm showed the best overall protective performance. The transparent elastic coating can effectively alleviate the hygrothermal degradation of GFRP and provide support for the surface protection design of flexible beam composite structures.