For application of epoxy adhesive to joining similar or dissimilar materials in vehicle bodies, its hygrothermal degradation (HTD) caused by severe environmental conditions of service will always be an issue until the relevant mechanism is clearly addressed and the remedy is found. This study provides experimental observations of an epoxy adhesive in terms of HTD and recoverability of the mechanical performance, and in the meantime, molecular dynamics (MD) simulations are performed to analyse the underlying mechanism. Comparing experimental results of the adhesive among states of the initial, HTD and dried manifests that the glass transition temperature (T-g) and the uniaxial tensile properties both reduced after HTD but partially recovered when dried. In the MD simulations, both of the dominant HTD factors, plasticization and hydrolysis, are accounted for via characterizations of water inclusion and bond scission. The simulation results reveal that both of the HTD factors reduce T-g, while only hydrolysis weakens the tensile properties. A quantitative comparison between the influences of plasticization and hydrolysis implies that hydrolysis is reversible for this specific epoxy adhesive.