In order to explore the change law of plant diversity in Pinus tabulaeformis and Picea meyeri stands at different stand ages and figure out the driving factors for plant diversity, we took Margalef index, Simpson index, Shannon-Wiener index and Pielou index as criteria for measuring biodiversity. We analyzed and compared the characteristics of biodiversity and soil physico-chemical properties of Pinus tabulaeformis stands and Picea meyeri stands at different stand ages in Youyi Reservoir, Erdaoying Basin, Xinghe County of Inner Mongolia. The results indicated that: 1) The biodiversity index of Pinus tabulaeformis artificial forest was greater than that of Picea meyeri artificial forest. The biodiversity of herbaceous community presented a nonlinear rising tendency with the increase of forest stand age. Five-year-old Pinus tabulaeformis forest had the highest biodiversity index of 2.09, whereas the biodiversity index of the thirteen-year-old forest was the lowest, only 1.63. The Margalef index of Picea meyeri artificial forest was higher than that of Pinus tabulaeformis forest, 3.79 in thirteen-year-old Picea meyeri and 2.16 in two-year-old Pinus tabulaeformis artificial forest. 2) During the process of vegetation restoration succession, perennial herbs replaced annual herbs gradually, ongoing towards a positive direction. 3) Among various edaphic factors, the content of soil water, soil anti-scouring ability and SOC had direct effects on biodiversity index. The results of our study could provide a theoretical support for vegetation restoration and rehabilitation in the forest-steppe ecotone.