To explore the effects of reduced light intensity on vitamin C content and enzymatic activities and sugar content in Rosa roxburghii Tratt. fruits and leaves. In this study, ‘Guinong 5’ R. roxburghii cuttings was taken as materials. The shading net was used to set three shading treatments with light intensity reduced by 20%, 40% and 60%, with natural light intensity as control. The changes of vitamin C content and related enzyme activities and sugar content in fruits and leaves of R. roxburghii under different light intensity were analyzed. The results showed that: (1) the content of vitamin C in fruits and leaves of R. roxburghii at different development stages were significantly decreased after the light intensity was weakened. The difference of vitamin C content in fruits of the control and the three treatments reached a significant level at the rapid expansion stage. (2) With the decrease of light intensity, the activities of Lgalactose1, 4lactone dehydrogenase (GalLDH), monodehydroascorbate reductase(MDHAR)and dehydroascorbate reductase(DHAR)decreased, while the activities of ascorbate oxidase(AAO)and ascorbate peroxidase(APX)increased in fruits and leaves of R. roxburghii, especially the activity of MDHAR increased rapidly and greatly in the rapid expansion stage. (3) The contents of soluble sugar, sucrose, glucose and fructose in the fruits and leaves of R. roxburghii decreased with the decrease of light intensity, while the contents of glucose increased rapidly during the rapid expansion stage. (4) The content of vitamin C in fruits was significantly positively correlated with sucrose and glucose contents in leaves at different development stages. At the rapid expansion stage, the activities of GalLDH and MDHAR in fruits were significantly positively correlated with vitamin C content in fruits, the APX activity was significantly negatively correlated with the vitamin C content in fruits, and their correlation coefficients are all at the maximum. The studied showed that light intensity effects the accumulation of vitamin C in R. roxburghii fruits by regulating the activities of key enzymes of vitamin C synthesis and related metabolism and the content of photosynthetic products in leaves; Weak light environment is not conducive to the accumulation of vitamin C and sugar in fruits and leaves of R. roxburghii, which is especially significant during the rapid fruit expansion stage; Good light was the basic condition to ensure the excellent quality of R. roxburghii fruits.