Increasing atmospheric nitrogen (N) deposition is an important driver of biodiversity change. By conducting an eight-year N addition experiment (0, 20, 50 and 100 kg N ha^-1 yr^-1), we investigated the long-term effect of simulated N deposition on understory species composition and richness in a boreal forest, northeast China. We found that moss cover decreased significantly with increasing N addition. N addition had no significant effect on vascular plants species richness but changed the plant community composition. The relative coverage of evergreen shrubs decreased, while that of graminoids increased under high-level N addition (100 kg N ha^-1 yr^-1). Under the high-level N treatment, Deyeuxia angustifoliacover increased significantly after 4 years, while that of Vaccinium vitis-idaea decreased significantly after 3 years and almost disappeared after 5 years. The negative effect of N addition on mosses and evergreen shrubs accumulated over time, while the positive effect on graminoids increased during the first 4 years and did not change significantly thereafter. Our results suggest that the effect of N deposition varies across functional groups and shifts over time.