In order to study the effects of nitrogen deposition on soil carbon and nitrogen fractions and enzyme activity in secondary forests, we took the moist evergreen secondary forests in rainy area of west China as the object. A simulated nitrogen deposition experiment in the field was conducted from January 2014. The addition levels of nitrogen included control (CK, +0 g/(m²·a)), low nitrogen (LN,+5 g/(m²·a)) and high nitrogen (HN,+15 g/(m²·a)). After 27 months of nitrogen deposition, soil samples of the humus layer and the surface of the leaching layer were collected to measure the contents of soil total organic carbon (TOC), extractable dissolved organic carbon (EDOC), readily oxidizable carbon(ROC), total nitrogen (TN), NO₃^--N, NH₄⁺-N as well as enzyme activities of sucrase, urease, acid phosphatase and polyphenol oxidase. The results showed that simulated nitrogen deposition significantly increased contents of TOC and NH₄⁺-N in the humus layer, and the contents of NO₃^--N in the humus layer and the surface of the leaching layer, and increased significantly the value of C/N in the humus layer of the secondary forest. The contents of TOC, NH₄⁺-N and C/N in the surface of the leaching layer and the contents of EDOC, ROC, TN and ammonium nitrate ratios in the two kinds of soil layers were not influenced by simulated nitrogen deposition. The polyphenol oxidase activity of the two soil layers decreased with the increasing of nitrogen addition, and the difference in surface of the leaching layer was significant. Simulated nitrogen deposition had no significant effect on the activities of invertase, urease and acid phosphatase. In the humus layer, there was a very significant positive correlation between the contents of NH₄⁺-N and NO₃^--N and the TOC. The polyphenol oxidase activities were significantly negatively correlated with the NO₃^--N content in the two soil layers. The results showed that the simulated nitrogen deposition further increased the TOC content in the humus layer of the secondary forest, and promoted the accumulation of soil inorganic nitrogen. The inhibition of simulated nitrogen deposition on polyphenol oxidase was more conducive to the accumulation of soil organic matter.