Abstract: To investigate the time-dependent degradation behavior and mechanical failure mechanism of SiO₂-modified epoxy adhesive under two typical marine atmospheric environments (cyclic corrosion and wet-dry cycling), thermogravimetric analysis (TG/DTG), fourier transform infrared spectroscopy (FTIR), mechanical tests (tensile and shear), and scanning electron microscopy/energy-dispersive spectroscopy (SEM-EDS) characterization were conducted. The results showed that the adhesive generally exhibited good thermal stability, featuring a single-stage main decomposition process. Aging did not alter the thermal degradation mechanism but accelerated the primary decomposition and reduced the residual carbon content. FTIR analysis revealed attenuation of the epoxy ring and enhancement of O—H, C=O, and C=C absorptions, indicating the coexistence of post-curing, oxidation, and moisture absorption effects. The mechanical behavior followed an evolution from initial strengthening to subsequent degradation: after 30 days of aging, post-curing led to a 7.60% increase in strength, while after 120 days, network damage and embrittlement caused a 13.44% reduction in shear strength. The tensile fracture mode changed from ductile to cleavage-dominated, and the shear failure transformed from cohesive to adhesive. Shear degradation was governed by the transport path of the environmental agents and interfacial structure, where the coupling of capillary infiltration and adhesive hygroscopicity weakened the interface and near-surface region. This study provides a reference for the durability assessment of modified epoxy adhesives in composite bonding systems under chloride-rich environments.