This study investigates the performance evolution of T800/polyaryletherketone（PAEK） thermoplastic composites subjected to hygrothermal aging conditions. By meticulously controlling the cooling rate, two distinct types of carbon fiber-reinforced PAEK composites with varying crystallinities are prepared: CF/PAEK-CL（low crystallinity） and CF/PAEK-CH（high crystallinity）. These composites are then systematically examined for their moisture absorption behavior, thermal properties, and mechanical performance in hygrothermal environments. Experimental results reveal that the water absorption of CF/PAEK composites increases progressively over time, with CF/PAEK-CL exhibiting a notably higher moisture uptake rate due to its lower crystallinity. Following hygrothermal aging, the glass transition temperature（T_g） of all samples decreases, with CF/PAEK-CL experiencing a specific reduction of approximately 5%. Thermal analysis further indicates that hygrothermal aging has a negligible impact on the crystallinity of the materials, and notably, the high-crystallinity composite demonstrates superior thermal stability in such environments. Flexural testing results demonstrate that hygrothermal aging has a limited influence on the flexural strength and modulus of CF/PAEK composites, underscoring their robust resistance to the detrimental effects of hygrothermal conditions on flexural mechanical properties. This resilience ensures their long-term stability and reliability in harsh environments. The findings of this study offer pivotal data and theoretical insights, paving the way for the application of CF/PAEK composites in demanding service conditions.