Gray mold caused by Botrytis cinerea leads to severe postharvest losses in fruit crops, and green alternative control strategies are urgently needed. E-2-Octenal, a plant-derived aliphatic aldehyde, exhibits antifungal activity against the postharvest pathogen B. cinerea, yet its mode of action remains poorly understood. Here, we investigated the antifungal mechanism of E-2-Octenal and evaluated its efficacy in postharvest fruit. E-2-Octenal suppressed mycelial growth and spore germination in vitro, and effectively controlled gray mold on strawberry and grape fruits. Transcriptomic analysis revealed that E-2-Octenal activated oxidative stress responses and protein degradation pathways. Molecular docking and biochemical assays demonstrated that E-2-Octenal directly bound to catalase Bccat7 and reduced its activity. Furthermore, E-2-Octenal promoted ubiquitin-mediated degradation of Bccat7. These effects exacerbated intracellular H₂O₂ accumulation and triggered reactive oxygen species (ROS)-mediated oxidative damage in the pathogen, thereby contributing to inhibition of B. cinerea growth. Our findings identify catalase Bccat7 as a target of E-2-Octenal and provide a mechanistic basis for its application in postharvest disease management.