【Objective】This study investigated the effects of different nitrogen (N) fertilizer concentrations on lignin content and the activities of key enzymes involved in lignin biosynthesis in the stems of Styrax tonkinensis seedlings. This study aims to provide a theoretical and practical basis for improving the papermaking quality of S. tonkinensis wood through nitrogen management.【Method】Three-year-old S. tonkinensis seedlings were subjected to five N application concentrations: 0 (CK), 1.5 (N1), 3.0 (N2), 6.0 (N3), and 9.0 (N4) g per plant. Fertilizer was applied on the afternoons of June 15, July 5, and July 25. Stem samples were collected on June 15 (pre-treatment), July 15, August 15, and September 15 to determine lignin content, lignin monomer composition (G, S, H types), and the activities of key lignin biosynthesis enzymes.【Result】The lignin content in the stems showed an initial decrease followed by an increase over time. On July 15 and August 15, the N1 treatment significantly increased lignin content, whereas the N2, N3 and N4 treatments resulted in lower content than that of the CK and maintained consistently low levels. The contents of G-and S-type lignin monomers generally increased then decreased with rising N concentration, while H-type monomer content showed no clear trend. The S/G ratio was higher under the N1 and N2 treatments on July 15. Within a certain range, low N concentrations enhanced the activities of phenylalanine ammonia-lyase (PAL), peroxidase (POD), cinnamic acid dehydrogenase (CAD), and trans-cinnamic acid-4-monooxygenase (C4H), whereas high concentrations suppressed them. Correlation analysis revealed a significant positive correlation between lignin content and POD activity on July 15 (P < 0.05), and negative correlations with CAD, C4H and PAL activities. During the last three sampling dates, POD activity remained positively or highly significantly positively correlated with lignin content. On July 15 and August 15, the S/G ratio showed a significant negative correlation with G-type monomers and a significant positive correlation with S-type monomers.【Conclusion】Increased nitrogen application during the fast-growing stage of S. tonkinensis reduce the activities of key lignin biosynthesis enzymes and decrease lignin synthesis and accumulation. Applying N at the N2 concentration (3.0 g/plant) in the early fast-growing stage reduces lignin content of stems while increasing the S/G ratio, thereby improving papermaking performance.