Abstract: Long-term seawater immersion degrades the mechanical performance of composite materials in marine service, which undermines the durability and safety of related structures. In this work, the moisture absorption and mechanical behavior of a typical ship composite sandwich structure with thick-section was investigated by experiments and simulations. First, the synchronous aging tests were designed for both composite material and sandwich structure. The evolution of moisture and mechanics characteristics of the panel composite during seawater immersion were quantified. The bending strength of this kind of thick-section structure at different aging days was measured, revealing its representative failure mechanism. Second, accounting for degradation diversity of sandwich components (panel, core and adhesive) under seawater aging, a finite element model was developed to characterize the moisture diffusion and mechanical response of the specimen. Moisture concentration, stress/deformation distribution and damage propagation were numerically analyzed. Overall, the bending failure loads and modes at different aging days captured from structural model simulation have good agreement with test results. Finally, quantitative comparison demonstrated that degradation of adhesive properties due to moisture absorption substantially reduced the bearing capacity of such sandwich structure for bending loads, but could not affect its crucial damage mode. Debonding at the adhesive layer between panel composite and core material remained the key factor for the failure of investigated specimen, both before and after aging.