Abstract: Polyvinyl alcohol (PVA) is considered a prime candidate material for replacing polyvinylidene chloride (PVDC) coatings due to its good transparency, superior oxygen barrier properties, and excellent film-forming ability. Currently, compared to other environmentally friendly high-barrier transparent films, PVA demonstrates compatibility with existing coating equipment designed for PVDC. This compatibility alleviates the pressure for the widespread market replacement of equipment, offering significant cost-effectiveness. However, PVA suffers from high moisture sensitivity and poor thermal stability, rendering pure PVA coatings inadequate for demanding high-barrier applications. To address this limitation, this study employed polyacrylic acid (PAA) as a modifier to prepare PAA/PVA composite coatings with various PAA contents under mild conditions via a facile and efficient polymer blending and crosslinking method. Glutaraldehyde (GA) was subsequently introduced into the composite coating, and PAA/PVA composite membranes were fabricated using a solution-casting technique. The chemical structure, morphology, and properties of the heat-treated PAA/PVA composite membranes were characterized. Finally, the composite coating membranes was blade-coated onto polyethylene terephthalate (PET) films, and the barrier properties and optical performance of the coated membranes were evaluated to investigate the role of PAA in mitigating the humidity sensitivity of PVA coatings and its impact on oxygen barrier performance. The results demonstrate that esterification crosslinking occurs between PAA and PVA at 120℃, synergistically enhanced by hydrogen bonding networks. This significantly improves the water resistance of the composite coating and effectively mitigates the deterioration of oxygen barrier performance under high-humidity conditions. The incorporation of GA further crosslinks the hydroxyl groups of PVA molecular chains, enhancing the water resistance of the composite coating while simultaneously improving the thermal stability and mechanical properties of the membranes. The composite coating exhibits excellent wettability on PET substrates, forming a transparent and uniform layer after drying. Moreover, the PAA/PVA-coated PET films (PET-PAA/PVA composite coating membranes) show minimal fluctuation in oxygen transmission rate (OTR) under dry conditions, while its oxygen barrier performance is significantly enhanced under high humidity. The addition of GA further reduces the OTR of the PET-PAA/PVA composite coating membranes. Compared to the PET-PVA coating membranes, the PET-10%PAA/PVA composite coating membranes exhibit reductions in OTR of 20.8% and 91.5% at 0% RH and 85% RH, respectively. This study aims to elucidate the mechanism by which PAA improves the humidity sensitivity of PVA coatings and its influence on oxygen barrier performance, providing a theoretical foundation for its industrial application in food flexible packaging.