Abstract: The phenylpropanoid pathway, one of the important secondary metabolic pathways in fruits and vegetables, can produce a wide range of phenolic substances, which have many biological activities, such as antioxidant, antibacterial and immunity enhancing. The type and content of produced phenols determine the flavor and quality of fruits, particularly on the nutrition and health. Since L-cysteine is a typical amino acid in living organism, previous studies have found that exogenous L-cysteine treatment can effectively delay the senescence and quality loss of plum fruit during storage after harvest. However, there are few reports on the effect of L-cysteine treatment on the synthesis of phenolic compounds in fruit. Taking 'Qingcui' plum fruit as the test material, this study aims to investigate the effect of L-cysteine treatment on the phenylpropanoid metabolism pathway, in order to provide theoretical support for the shelf life of fruit and preservation during postharvest storage. Specifically, the plum fruit was soaked with L-cysteine solution at 1 g/L for 10 min, and then stored at (20±1)℃ with 85%-90% relative humidity. The effect of L-cysteine treatment on key enzymes activities in phenylpropanoid pathway was investigated, including phenylalanine ammonia lyase (PAL), 4-coumaric coenzyme A ligase (4CL), and cinnamate-4-hydroxylase (C4H), as well as the change rule of total phenols, flavonoids and other metabolites. The antioxidant activity of plum fruit was also evaluated. The results showed that L-cysteine treatment significantly(P<0.05) delayed the decrease of total soluble solid and titratable acidity content of plum fruit during postharvest storage, indicating that can maintain an excellent quality of fruit. Moreover, the activities of key enzymes increased gradually in the phenylpropane metabolic pathway during storage. The activities of PAL and 4CL of plum fruit in the treatment group were higher than that in the control group. Compared with control group, L-cysteine treatment can increase the content of total phenols and total flavonoids significantly in the first three days of storage, where the content decreased first, and then increased. In the determination of phenolic monomers, protocatechuic acid, p-coumaric acid, chlorogenic acid, and caffeic acid decreased first and then increased during storage, while syringic acid and rutin increased gradually. The contents of phenolic monomers in the treated fruits, such as chlorogenic acid, caffeic acid, syringic acid, and rutin, were significantly higher than that in the control group(P<0.05). The trend of antioxidant activity was consistent with that of total phenols and flavonoids, while the fruits maintained high antioxidant activity during storage after L-cysteine treatment. The correlation analysis revealed that the activities of PAL, 4CL and C4H enzyme in fruit were significantly correlated to the content of phenolic substances and antioxidant capacity (P<0.05), whereas, the antioxidant activity in the fruit was extremely significantly correlated with total phenols, total flavonoids and other metabolic substances (P<0.01). These findings demonstrated that 1 g/L L-cysteine treatment can efficiently activate the phenylpropanoid pathway of fruit, thereby to promote the accumulation of phenolic substances. Therefore, the L-cysteine treatment can effectively enhance the storage quality of 'Qingcui' plum fruit.