A pot experiment was conducted to investigate the effects of biochar (BC) amendment (0, 12, 36 t/hm²) on soil organic carbon (SOC) and its fractions in a Cunninghamia lanceolata seedling under different simulated levels of nitrogen (N) deposition (0, 40, 80 kg/(hm²·a) N) for 18 months. The results indicated that compared with the control, BC amendment significantly increased the contents of SOC, recalcitrant pool of carbon (RPC), and dissolved organic carbon (DOC) by 40.1%~99.2%, 104.0%~267.8% and 75.3%~194.7% under different levels of N deposition (P<0.05), respectively. Meanwhile, the proportion of labile C from acid hydrolysis (LPC) in SOC decreased significantly by 28.8%~53.3%, and the proportion of RPC in SOC increased significantly by 45.7%~84.6% in the treatments of BC amendment combined with N deposition (P<0.05). The effects were the most significant when higher-rate BC was applied alone (P<0.05). No significant differences were found in the content of LPC in the treatments of BC amendment alone or simulated N deposition alone, whereas LPC was significantly reduced under the higher BC amendment combined with simulated N deposition (P<0.05).The higher rate of simulated N deposition alone significantly decreased soil microbial biomass carbon (MBC) content and the proportion of MBC in SOC, but had no significant effect on the DOC content and the proportion of DOC in SOC. However, the interaction of simulated N deposition and higher rate of BC significantly increased the proportion of DOC in SOC (P<0.05). Correlation analysis suggested that soil pH and available nutrients (AP, AK) were positively correlated with SOC, RPC and DOC contents, and were significantly negatively correlated with LPC (P<0.01). Our results indicated that the interaction between simulated N deposition and BC amendment has the greatest impact on RPC fraction, which is conducive to improving the stability of SOC fractions and increasing soil C storage in the short term.