To study the effects of magnesium(Mg) fertilizer on fruit yield and quality of Satsuma Mandarin (Citrus unshiu Marc.), so as to provide a theoretical basis for the rational application of Mg fertilizer. The experiment was set up in 2017, with 20-yr-old Satsuma Mandarin trees in Yiling District, Yichang City, Hubei Province. Five treatments were designed as CK, T1, T2, T3 and T4, Mg application doses were MgO 0, 100, 150, 200 and 250 g/tree each year. The sugar and organic acid contents of pulp, and fruit colouration were determined at fruit turning color period and fruit maturity from 2019 to 2020. Compared with treatments without Mg applications, Mg fertilizer significantly increased the fruit yield of Satsuma Mandarin, and the fruit yield increased first and then decreased with the increase of Mg fertilizer dosage. In 2019 and 2020, the fruit yield was the highest when the Mg fertilizer dosage was 124 g MgO / plant and 122 g MgO/ plant, respectively. At the fruit turning color period, Mg application increased the sucrose content in the pulp , significantly increased the L, a, b and C values of pericarp, significantly reduced the H value, chlorophyll a, chlorophyll b and total chlorophyll contents of pericarp, improved the brightness, chroma and purity of pericarp. However, there was no significant effect on sugar and acid contents of pulp, the L,a, b, C values and chlorophyll content of the peel during fruit maturity. The results of principal component analysis showed that the fruit internal and external quality of T2, T3 and T4 treatment was better, while the fruit quality of T4 treatment was better from the perspective of principal component score.The Mg application rate of Satsuma Mandarin with the highest yield and better quality is 122～150 g MgO/plant, and the Mg application rate for the best quality is 200～250 g MgO/plant.