The Great Khingan Mountains, located in northeastern China, feature zonal permafrost. Lying on the southern edge of the Eurasian permafrost boundary, the permafrost region of the northeast portion of the Great Khingan Mountains is one of the plant communities most sensitive to the effects of global warming. Permafrost is gradually degrading from south to north, because of the effects of human activity and environmental changes such as global warming. The degradation of permafrost leads to changes in the characteristics of permafrost plant communities, such as species composition, species diversity, and biomass. The impacts of permafrost on vegetation have become a key research topic in the field of climate change research. However, previous studies of permafrost have mostly focused on the distribution of permafrost, and so the mechanistic connection between permafrost and environmental factors, and the impacts of permafrost degradation on community composition and structure have not been the focus of much quantitative research. Thus, the plant communities of 30 plots on the north slope of the Great Khingan Mountains were classified according to permafrost depth, using two-way indicator species analysis (TWINSPAN) and canonical correspondence analysis (CCA). Accordingly, the effects of permafrost depth on plant species diversity were analyzed using these plots. The results indicate the following. (1) Plant communities were categorized into three plant associations, and as the permafrost depths changed along a gradient from shallow to deep, plant associations changed from Betula fruticose+Ledum palustre var. angustum-Carex subpediformis to Betula fruticosa-Carex subpediformis and Betula fruticosa+Salix rosmarinifolia-Carex subpediformis. These relationships were validated by CCA ordination. In addition, permafrost melting depth (PMD), topography, and soil moisture could together explain 38% of the community change. Their combined effect was the biggest explanatory factor for community change. (2) The species diversity index increased trend initially (in shallow permafrost) and decreased as the permafrost melting depth increased. It reached its maximum value at about 50-150 cm deep. When PMD≤50 cm and PMD > 150 cm, the species richness and diversity index are low, and the significant difference. Patrick index and melting permafrost depth has significant correlation (R²=0.58, P<0.58). When PMD≤50 cm, the species richness of the communities was higher, at 23.83±2.44; when 50 < PMD≥150 cm, it was 26.36±2.01; and when PMD >150 cm, it fell sharply to 21.14±1.57. The Pielou index and Shannon-Wiener index with the increase of the permafrost melts deep appeared a trend of reducing the rise, and when 50 < PMD < 150 cm, the Pielou index and Shannon-Wiener index value were significantly higher than those of other permafrost melts deep under the index value. The Simpson index values fell significantly when PMD >150 cm. This study provides a theoretical basis for the prediction of trends in the variation of plant community composition after the degradation of permafrost. It is also important for the development and implementation of agriculture and forestry in permafrost areas. In addition, it would be an effective guide for forest management and biodiversity conservation efforts in permafrost regions.