Nitrogen deposition is one of the important driving factors of biodiversity change. To explore the effect of nitrogen addition on the understory plants diversity of the Korean pine (Pinus koraiensis) plantation and its driving mechanism, we simulated N deposition with four levels of N addition rate N0 (control treatment, 0 kg hm^-2 a^-1), N20 (low nitrogen treatment, 20 kg hm^-2 a^-1), N40 (medium nitrogen treatment, 40 kg hm^-2 a^-1) and N80 (high nitrogen treatment, 80 kg hm^-2 a^-1) for 6 years, based on the Korean pine plantation in Liangshui National Nature Reserve in Heilongjiang Province. The results showed that:(1) nitrogen addition significantly reduced the density and coverage of the three functional groups in the understory plants, but had no significant effect on the height. (2) The 6-year nitrogen addition reduced community similarity between control and nitrogen application treatment decreased with the increase of nitrogen application level. (3) Nitrogen addition significantly reduced the Shannon-Wiener diversity index and species richness of herbs in the Korean pine plantation. Nitrogen addition had no significant effect on the species richness and Shannon-Wiener diversity index of woody plants and ferns. Nitrogen addition had no significant effect on the Pielou evenness index of the three functional groups of herbs. (4) Nitrogen addition had significant effects on the contents of C, N, P, N:P and C:P of herbs. Nitrogen addition had significant effect on the contents of P, N:P and C:P of woody plants, and it had no significant effect on C:N:P ecological stoichiometry of ferns. (5) There was no significant correlation between herb diversity and soil chemical properties. There was significantly positive correlation between species richness of herbs, Shannon Wiener index and coverage, density of herbs. There was significantly negative correlation between species richness of herbs and plant N content, and between herb Shannon Wiener index and plant N:P. The 6-year nitrogen addition changed the species composition and community structure in the understory plants, significantly reduced the density and coverage of the three functional groups, and had no significant change on the height, and it only reduced the species richness and Shannon-Wiener diversity index of herbs. The reason for this phenomenon may be that different species have different nitrogen utilization characteristics and tolerance, and nitrogen addition causes nutrient imbalance of herbs, changes species composition and community structure, and affects herbs diversity. The results provide data and theoretical basis for the sustainable management in temperate forest ecosystems in China.