Toona sinensis (TS) is a widely cultivated and economically valued tree in Asia. However, millions of tons of its nutritious mature leaves go unutilized due to excessive amount of nitrate and nitrite. In this study, the mature leaves of TS were chopped, vacuum-packed, and fermented for up to 18 weeks to evaluate their feeding potential. The dynamic effects of fermentation on quality indicators, including crude protein, flavonoids, nitrate and nitrite content, and antioxidant activity [radical 2,2-diphenyl-1-picrylhydrazyl (DPPH) scavenging activity, ferric-reducing antioxidant power (FRAP) and oxygen radical absorbance capacity (ORAC)] of mature leaves of TS were quantified every two weeks. Results showed that after fermentation for 6 weeks, the total content of nitrate and nitrite of TS leaves was significantly decreased (p ≤ 0.05) from 4523.36 to 1102.20 mg/kg. The mean content of nutritional components, including crude protein (17.88 ± 0.49%), flavonoids (792.24 ± 50.78 mg/100 g), and antioxidant activity (317.62 ± 44.70 mg/100 g for DPPH, 400.44 ± 25.87 mg/100 g for FRAP, 473.74 ± 37.17 μM/g for ORAC) of TS leaves were well preserved between 6 to 14 weeks of fermentation. These results indicated that fermentation may provide a new way to utilize TS resources, and the fermented mature leaves of TS could be a novel, valuable feed supplement, especially for ruminants. Screening and inoculating TS leaves with an appropriate starter culture, such as lactic acid bacteria inoculants, during fermentation is recommended to furtherimprove TS feed quality.