Biodiversity conservation including plant species in natural forests is very important for their ecosystem service function. Daxing'anling is an important state-owned forest region in China. The characteristics and coupling relationship of the existing national nature reserves are the basis for the comprehensive improvement of natural forest protection and ecological services. In this paper, from August to September of 2017, the individual size characteristics, community characteristics and plant diversity characteristics of arbor, shrub, herb and regeneration layer were investigated in Duobukuer National Nature Reserve. The redundant analysis and variance partitioning analysis were conducted to analyze the coupling association and possible factors responsible for diversity variations. The results showed that:(1) Larix gmelinii is the main tree species in Duobukuer, had a relative abundance of 42%, followed by Betula platyphylla and Betula dahurica. The proportion of hazelnut (Corylus heterophylla) is the highest among shrubs, reaching 60%. A total of 116 species of herb layers were found, of which Carex callitrichos accounted for the largest proportion, exceeding 10%. The main tree species in the regeneration layer was Quercus mongolica (54%), followed by Betula platyphylla. (2) The diameter and height of trees were only 55% of those in 1970s, and the richness of trees and herbs was about 50%-60% higher than that in Huzhong Nature Reserve of the central Daxing'anling. (3) Redundancy analysis and variation partitioning showed that the individual size (rather than community features), and arbor layer characteristics (rather than shrub and herb layer) contributed the most to the plant diversity variation observed in this study. Of all the tested parameters, tree height gave the most significant explaining power for the plant diversity variations. The forests in Daxing'anling Mountain have been fully included in the Natural Forest Protection Program (NFPP, completely inhibited timber harvest since 1999). The above-mentioned data from the National Nature Reserve, as a local background data, clearly showed that the forests in this region were still in their young stage and long-term protection are necessary. These data will provide supports for further national reserve policy making and implementation of the NFPP in northeast China.