Abstract: Carbon fiber/vinyl ester (CF/VE) composites are widely used in wind energy and marine engineering applications; however, their long-term service performance is prone to degradation under hygrothermal environments. Although previous studies have demonstrated that resin moisture absorption and swelling induced by hygrothermal aging lead to mechanical property degradation, quantitative investigations linking moisture-induced swelling to mechanical degradation remain limited. To address this issue, an integrated research framework was established to characterize moisture diffusion, hygroscopic swelling strain, and mechanical property degradation of CF/VE composites under hygrothermal conditions. CF/VE laminates with different lay-up designs were fabricated and subjected to accelerated hygrothermal aging in artificial seawater and deionized water at 70℃. Based on a one-dimensional Fickian diffusion model and gravimetric measurements, the saturated moisture content M_∞ and diffusion coefficient D were determined for various specimens, with the maximum D reaching approximately 2.83 × 10⁻⁶ mm²/s in artificial seawater. Embedded fiber Bragg grating (FBG) sensors were employed to achieve in situ monitoring of internal hygroscopic swelling strain in the laminates. Pronounced anisotropy in the coefficient of moisture expansion (CME, β) was observed for unidirectional laminates: β in the 0°direction was approximately 1–1.6 × 10⁻⁶, whereas that in the 90° direction reached as high as 26–42 × 10⁻⁶. Meanwhile, longitudinal/transverse tensile, longitudinal/transverse compressive, and in-plane shear tests were conducted to investigate the evolution of mechanical properties during hygrothermal aging. The results indicated that after 90 days of aging, the transverse tensile strength decreased by approximately 24%–25%, while the in-plane shear strength decreased by about 17%–18%. Based on the obtained diffusion and hygroscopic swelling parameters, finite element analysis was performed to determine the internal hygroscopic swelling strain field of the laminates, and the numerical results were validated against the FBG measurements. This study achieves a coupled characterization of hygrothermal diffusion, internal hygroscopic swelling strain, and mechanical degradation, providing a systematic analytical methodology and theoretical basis for the service life prediction and structural design of CF/VE under hygrothermal environments.