With the development of agriculture and industry, the increase in nitrogen (N) deposition has caused widespread concern among scientists. Although emission reduction policies have slowed N releases in Europe and North America, the threat to biodiversity cannot be ignored. Arbuscular mycorrhizal (AM) fungi play an important role in the establishment and maintenance of plant communities in forest ecosystems, both their distribution and diversity have vital ecological functions. Therefore, we analyzed the effects of long-term N addition on AM fungi and understory herbaceous plants in Korean pine plantation in northern China, measured the soil properties, AM fungal and herbaceous community structure and diversity with different concentrations of NH4NO3 (0,20,40,80 kg N ha-1 year-1) applied for 7 consecutive years. The results showed that long-term nitrogen fertilization affected soil properties, decreased soil pH, increased soil ammonium content, and caused significant fluctuations in P elements. Nitrogen application improved the stability of soil aggregates by increasing the content of GRSP (glomus-related soil protein); N addition changed the AM fungal community composition, and Glomus genus was more adaptable to the acidic environment treated with the highest nitrogen concentration; with the increase of N treatment, The species of AM fungi and herbaceous plants and the biomass of fine roots decreased. In summary, we concluded that long-term understory application of different concentrations of N altered soil pH, changed the distribution of N, P elements, and the soil aggregate fractions, reduced AM fungal and herb diversity. The importance of AM fungi in maintaining forest ecosystem diversity under the pressure of global change was verified.