Soil extracellular enzymes play a key role in mediating the degradation of soil organic matter, but little is understood as to how the pattern of soil extracellular enzymes could be altered by nitrogen (N) addition. In this study, the effects of N addition (started from 2006, four treatments: control, 0 g N·m^–2·yr^–1; low N addition, 2.5 g N·m^–2·yr^–1; high N addition, 5.0 g N·m^–2·yr^–1) on soil extracellular enzymes and microbial biomass in a natural secondary forest of Northeastern China. The results showed that the activity of urease, sucrase, peroxidase and N-acetyl-β-D-glucosidase decreased with N addition, and the activity of acid phosphatase and leucine aminopeptidase increased significantly with N addition. Soil total N, temperature at 5 cm soil depth, pH value, microbial biomass carbon and microbial biomass N were the key factors affecting soil enzyme activity. In summary, the enzyme activity related to soil organic matter degradation shows a decreasing trend under N addition. The results suggest that the increase in N deposition will slow down the degradation of soil organic matter in natural secondary forest, which is more conducive to the accumulation of organic matter in the soil.