In order to grasp the change trend of understory plant composition, diversity and interspecific relationship of broad-leaved Korean pine forest in the southern part of the Xiaoxing"an Mountains at different succession stages (pioneer, middle, stable and top stages). Based on the field survey data, four forest plant communities, pioneer community (natural maple birch secondary forest), middle community (secondary broad-leaved forest), stable community (secondary coniferous and broad-leaved mixed forest) and top community (primitive broad-leaved Korean pine forest) composed of the succession sequence of pioneer stage (I), middle stage (II), stable stage(III), and top stage were taken as the research objects. Differences in species diversity indexes and niche characteristics in shrub layers. The results showed that: (1) There were 48 species of herbaceous plants in 37 genera and 27 families, and 18 species of shrub plants in 13 genera and 10 families. The importance values of Aegopodium alpestre in the I, II, II, III stages of succession were the highest (17%, 19%, 17%), but the importance value of Oxalis griffithii were the highest(16%) in the Ⅳ stage.The importance value of Deutzia scabra in the shrub layer increased gradually during the succession process, and the importance value was the highest in each stage (25%, 28%, 33%, 37%). (2) During the succession process, the diversity indices of the four α herbaceous and shrub plants showed a "unimodal" response pattern, and except for the Margalef richness index of herbaceous plants, which was significantly different in succession, there was no significant difference in other diversity indexes. The variation range of the diversity index of the two species β of herbaceous plant communities (0.35~0.66 and 0.75~0.85) was higher than that of shrub communities (0.24~0.39 and 0.72~0.77) in each succession stage, and the community similarity between the two communities in the succession sequence was not necessarily the highest. (3) In the herbaceous layer, Herbaceous layer of Aegopodium alpestre had a high niche width (4.91, 4.35, 6.15) and importance values (17%, 19%, 17%) in the succession I, II, III, occupying a high position and dominance in the community, but in the succession IV stage, its niche width (3.62) and importance value (7%) began to decline, and the status and dominance were gradually replaced by Athyrium brevifrons and Oxalis griffithii . Shrub layer of Deutzia scabra occupies the largest niche width (7.44, 7.98, 8.03, 6.36) and importance values (25%, 28%, 33%, 37%) at each stage, and has strong resource utilization capacity and environmental adaptability. Herbaceous plants and shrubs accounted for a large proportion of O_ik≤0.4 in each succession stage (79.17% in the herbaceous layer and 64.29% in the shrub layer), and the degree of niche overlap was low, and the understory plants were not prone to interspecific competition, and the community was relatively stable.