<p>The development and application of biodegradable materials are an effective strategy for reducing environmental pollution and promoting sustainable development. However, there are limitations in the processing, mechanical, and barrier properties of these materials, which urgently need to be improved. The addition of additives is one of the most convenient and effective methods for obtaining high-performance polymer materials. However, there are few varieties of special modifiers for biodegradable materials, which are poorly targeted and have a single function, resulting in a small performance enhancement of biodegradable materials and high processing-and-moulding difficulties. In this study, in response to the characteristics of biodegradable materials (which generally have a low polymerization degree and require chain expansion during processing), epoxy functional additives (A-QR) with active components were successfully prepared using the ring-opening reaction between the hydroxyl and epoxy groups on quercetin. The results showed that the incorporation of A-QR improved the processing, thermal stability, crystalline, mechanical, and barrier properties of the films. In particular, when 0.6% A-QR was added to the PLA films, the tensile strength increased by 38.2%, and the water vapour transmission rate decreased by 18.9%. In addition, UV spectral analysis confirmed that A-QR conferred excellent UV shielding and antioxidant properties to the PLA films, with the UV shielding efficiency of the PLA/A-QR composite film reaching 79.24%, and the free radical scavenging rate increasing to 83%. Reactive extrusion technology enables effective immobilization of A-QR in PLA and limits its migration during use. The PLA/A-QR composite film demonstrated excellent freshness retention in food packaging applications.</p>