To address unpredictable flavor changes in postharvest peaches during storage, this study investigated the use of bioactive packaging with melatonin-infused P34HB films. Films with melatonin concentrations of 0%, 1%, 3%, and 5% were prepared using the extrusion casting method and applied to peach storage at room temperature. Comprehensive film properties were characterized, showing that melatonin minimally impacted the films' mechanical properties, including gas and water vapor permeability, but significantly increased film haze. Using GC-IMS, 30 organic compounds affecting peach flavor were effectively identified, including 8 aldehydes, 5 alcohols, 4 ketones, 12 esters, 1 pyrazine, 1 olefin, and 1 furan. Unpackaged, naturally ripening peaches served as a reference for assessing flavor and quality changes across various packaging groups during storage. The results indicated that the appearance of off-flavor organic compounds, such as ethanol produced by peach anaerobic respiration and complex esters, was the primary cause of flavor deterioration. The P34HB film with 1% melatonin most effectively preserved the original flavor and juiciness of peaches, highlighting its potential as an active packaging solution for fruit.