Thirty-six leaf samples of Sophora davidii, Indigofera amblyantha, and Broussonetia papyrifera were collected during different growth periods. The dynamic changes in nutritional components and secondary metabolites were analysed, and the biological activities of the secondary metabolites were evaluated. The optimal utilization stage of each plant was comprehensively determined. The results showed that S. davidii (the crude protein (CP) content 24.82%, relative feed value (RFV) 186.52) and I. amblyantha (CP 24.28%, RFV 177.54) were premium forage materials and that B. papyrifera (CP 21.65%, RFV 133.84) was a first-level forage. The best period for using the leaves of these three plants as animal feed was the flowering stage. As the growth progressed, the contents of CP, crude fibre (CF), dry matter (DM), and nitrogen-free extract (NFE) decreased, while the CF, neutral detergent fibre (NDF), acid detergent fibre (ADF), and crude ash (Ash) increased.