With the nectarine variety Shuguang as test material and blue light set artificially, the photosynthetic performance and the accumulation of sugar and acid in leaves and fruits were examined, and diurnal changes of the stomata in five key growth periods were observed. Blue light increased the net photosynthetic rate and the contents of chlorophyll a and b. The ratio of chlorophyll a/b was decreased and the leaf area increased. The opening time of the stomata was advanced, and maximum opening was earlier and closing time hysteretic. Fructose, glucose and sorbitol were the main assimilates in leaves, with the highest content being sorbitol. Fructose, glucose, sorbitol and sucrose were the main assimilates in fruit, and sucrose was the main soluble sugar at fruit maturity. As compared with the control, blue light conferred lower content of the three assimilates in leaves, whereas the content of total sugar and sucrose in fruit was higher, which suggests that blue light enhanced the ability to transfer photosynthetic products from leaves to fruit. Oxalic acid was the main organic acid in both leaves and fruits. The content of organic acid under blue light in leaves was decreased, and the sugar to acid ratio was significantly increased, by 30.5%. The key period of peaches improving the sugar to acid ratio under blue light was the late hard nucleus period. Supplementing blue light could be a technical measure to improve photosynthetic performance and fruit quality and could be further applied in the cultivation of greenhouse fruit trees.