20-year-old satsuma mandarin trees in Yiling District，Yichang City，Hubei Province were used to study the effects of Mg fertilizer on the components of sugar and acid in pulp，the value of color difference and content of pigment in peel at the stage of fruit color changing and fruit maturity of satsuma mandarin to balance the rational application of magnesium fertilizer in citrus and improve the internal and external quality of citrus fruit.Five magnesium fertilizer levels were set as 0，100，150，200，and 250 g MgO per plant，denoted as CK，T1，T2，T3，and T4.The results showed that magnesium application significantly increased the content of magnesium in leaf at the stage of fruit expansion compared with no magnesium application.Mg fertilizer significantly increased the yield of satsuma mandarin fruit.The yield of fruit increased first and then decreased with the increase of Mg fertilizer dosage. In 2019 and 2020，the yield of fruit was the highest at 124 g and 122 g MgO per plant，respectively.Magnesium fertilizer increased the content of sucrose in the pulp at the stage of fruit color changing.Mg application significantly increased the value of L，a，b and C in peel，significantly reduced the H value，the content of chlorophyll a，chlorophyll b and total chlorophyll in peel.The brightness，chroma and purity of peel was improved.However，there was no significant impact on the content of sugar and acid in pulp，the value of L，a，b，C and the content of chlorophyll in peel at the stage of fruit maturity.The results of principal component analysis showed that the overall internal and external quality of fruit in T2，T3 and T4 treatment was better，while the quality of fruit in T4 treatment was the best at the stage of fruit coloring.It is indicated that the recommended magnesium application rate for the highest fruit yield and better quality of satsuma mandarin is 122-150 g MgO per plant，and the recommended magnesium application rate for fruit quality of satsuma mandarin is 200-250 g MgO per plant.