Abstract: This study investigated the effects of exogenous cinnamic acid (CA) on reddening and reactive oxygen species (ROS) metabolism in fresh-cut lettuce. Fresh-cut lettuce samples were treated with 0.5 g/L CA solution for 10 minutes (CA group), while distilled water was used as a control (CK group). Both groups were stored at 4±0.5 ℃, and samples were collected every two days over a period of 8 days. The results showed that CA treatment significantly suppressed the increase in reddening index and a^* value, and mitigated the decline in L^* value, thereby maintaining the visual quality of the lettuce during storage. Furthermore, CA treatment markedly inhibited the activities and gene expression levels of polyphenol oxidase (PPO), peroxidase (POD), and phenylalanine ammonia-lyase (PAL), delaying the biosynthesis and oxidation of total phenols, flavonoids and scopoletin. In addition, CA treatment maintained higher activities and expression levels of antioxidant enzymes, including superoxide dismutase (SOD), catalase (CAT), glutathione reductase (GR), and ascorbate peroxidase (APX). It also reduced the accumulation of malondialdehyde (MDA) and hydrogen peroxide (H₂O₂), as well as the generation rate of superoxide anion (O₂^−.), thereby alleviating oxidative damage and preserving cell membrane integrity. Overall, this study provides theoretical support for the application of CA in the preservation of fresh-cut fruits and vegetables.