Selenium (Se) fertilizer has a good effect on many field crops, but there are few reports on the application of Se fertilizer on citrus. We investigated the effects of 0 mg/L (CK, water treatment), 50 mg/L, 100 mg/L, 150 mg/L, and 200 mg/L sodium selenite aqueous solutions on the growth, nutrition, and fruit quality of 15-year-old citrus unshiu (Citrus reticulata Blanco cv. Succosa). The results showed that a low concentration of Se fertilizer promoted the growth and development of the citrus plan, and a high concentration of Se fertilizer was found to slightly inhibit the growth and development of the plant. Among the different treatment groups, 150 mg/L selenium fertilizer showed have the best effect on these evaluated parameters. The results thus suggest that 150 mg/L of Se fertilizer promotes the formation of chlorophyll in the leaves of the test plant and increases the longitudinal and transverse diameter of the fruits and weight of single fruit, significantly enhancing the activity of antioxidant enzymes in the leaves, promoting the absorption of nutrients in the leaves, increasing the contents of total sugar and vitamin, and decreasing the acidity in the fruits and the pericarp thickness. It also promoted the accumulation of the total selenium content in the leaves and fruits and consequently improved the quality of the fruits. These results showed that appropriate concentration of Se treatment can improve the activity of antioxidant enzymes to enhance plant stress resistance, regulate the content of sugar and acid in fruits, and improve the quality of fruits.