Abstract: Dynamic changes in community species composition and functional diversity reflect the current status of a population and future trends of community succession. In this study, grassland vegetation under Robinia pseudoacacia L. stands with different ages was investigated to determine the characteristics of understory plant communities and plant functional traits. Results showed that the grassland understory was mainly composed of perennial weeds of Compositae and Rosaceae. With the increase in recovery period, the plant community gradually changed from one dominated by Compositae to one dominated by Gramineae. Furthermore, dicots gradually declined and monocots gradually increased. Except for total community coverage, the quantitative characteristics of the community showed a significant increase with restoration period. In addition, Rao’s quadratic entropy gradually decreased, while Functional richness (FRic), Functional evenness (FEve), and Functional divergence (FDiv) increased at first and then decreased with recovery years. Biomass and leaf traits became the main factors affecting functional diversity changes.