Global warming is an important problem faced by mankind at present. The increase in greenhouse gas emissions due to human activities is the main cause of global warming. The degraded agricultural land can reduce soil greenhouse gas emissions by improving the carbon sequestration capacity of ecosystems through vegetation restoration, but there is still uncertainty how plant community characteristics affect ecosystem carbon stocks following vegetation restoration. This paper selected a chronosequence of 0 (farmland), 8, 15, 25 and 35 years of grassland restoration since farmlands abandonment. We aimed to explore (1) how the plant community and ecosystem carbon stocks changed with the increase of restoration age; and (2) how the composition of the plant communities affected the changes of the ecosystem carbon stocks. The results showed that the community cover increased significantly with the increase of restoration age, and reached the maximum value (64.0%) in 35 years since farmlands abandonment. The advantage strains evolved from Lespedeza daurica, Leymus secalinus, Artemisia capillaris, to Stipa bungeana, Artemisia sacrorum; grasses, perennial herbs and shrubs gradually became the dominant species. The Shannon-Weiner index and the Patrick index both showed a trend of first increase and then decrease, reaching their highest level in the farmland abandonment for 15 years. The carbon stocks of aboveground vegetation and underground vegetation showed a linear increase during the restoration period. They both reached the maximum value in 35 years, which were 0.83 and 1.49 Mg C/hm², respectively. The carbon stocks of litters reached the maximum value of 0.40 Mg C/hm² in the 25 years. Soil carbon stocks and organic carbon content showed a trend of first decline and then increase, reaching the lowest value in 8 years and returning to the same level with farmland in 35 years, and soil carbon stocks accounted for 93.3%-99.6% of ecosystem carbon stocks in the grassland restoration ecosystems. Ecosystem carbon stocks were in line with soil carbon stocks, with a minimum of 24.32 Mg C/hm² in 8 years and 43.70 Mg C/hm² in 35 years. Community cover, above-ground biomass, litter, plant structure and function showed significantly positive correlation between carbon stocks (P<0.05), and the important values of forbs and annual herbs showed significantly negative correlation with ecosystem carbon stocks (P<0.05). The study suggested that vegetation community composition increased ecosystem carbon stocks by increasing vegetation and soil carbon stocks. The important values of perennials, forbs and grasses and underground biomasses and litters are important vegetation factors to affect the carbon stocks in the restoring grassland ecosystems.