To understand the trends in understory plant composition, diversity, and interspecific relationships in different succession stages (pioneer, middle, stable, and climax stages) of broad-leaved Korean pine forests in southern Xiaoxing'an Mountains, the method of space-for-time substitution was used to select four forest plant communities in the succession sequence of pioneer stage (Ⅰ) → middle stage (Ⅱ) → stable stage (Ⅲ) → climax stage (Ⅳ) as research objects: pioneer community (natural maple birch secondary forest), middle community (secondary broad-leaved forest), stable community (secondary coniferous and broad-leaved mixed forest), and climax community (primitive broad-leaved Korean pine forest). Based on the field survey data, the differences in species diversity indexes and niche characteristics of herbaceous layer and shrub layer of four forest plant community types (forest types) in different succession stages were analyzed. The results show that: (1) There are 48 species of herbaceous plants belonging to 37 genera and 27 families, and 18 species of shrub layer plants belonging to 13 genera and 10 families. Rosaceae is the dominant family of understory plants in this area. The important value of Aegopodium alpestre was the highest (17%, 19%, 17%) in the herb layer of succession stage Ⅰ, Ⅱ and Ⅲ, and the important value of Oxalis griffithii was the highest (16%) in the succession stage Ⅳ. The important value of Deutzia scabra in the shrub layer gradually increased during the succession process and was the highest value in each stage (25%, 28%, 33%, 37%). (2) In the process of succession, the four α diversity indexes of herbaceous and shrub plants showed a 'unimodal' response pattern. Except that the Margalef richness index of herbaceous plants was significantly different in succession, other diversity indexes did not show significant differences. The variation range of the two β diversity indexes 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), and the similarity between the two communities in the succession sequence was not necessarily the highest. (3) The niche breadth (4.91, 4.35, 6.15) and importance value (17%, 19%, 17%) of Aegopodium alpestre in the herb layer were higher in succession Ⅰ, Ⅱ, Ⅲ stages, which occupied a higher position and dominance in the community. However, in the succession Ⅳ stage, its niche breadth (3.62) and importance value (7%) began to decline, and the position and dominance were gradually replaced by Athyrium brevifrons and Oxalis griffithii. Deutzia scabra of the shrub layer occupied the largest niche breadth (7.44, 7.98, 8.03, 6.36) and important value (25%, 28%, 33%, 37%) in each stage, and had strong resource utilization ability and environmental adaptability. Species with O_ik (niche overlap index)≤ 0.4 accounted for a large proportion of herbaceous plants and shrub plants in each succession stage (average 79.17% of herbaceous layer, average 64.29% of shrub layer), and the degree of niche overlap was low. Interspecific competition among dominant species of understory plants was not easy to occur, and the community was relatively stable.