This study aimed to assess underground root biomass and carbon storage of forests of different ages and provide a basis for the management of forest operations and carbon sinks of Quercus acutissima forests. Quercus acutissima forests of different ages (young forests, half-mature forests, near-mature forests, and mature forests) were selected in Jurong, Jiangsu Province. The root excavation method was used to obtain the root systems of Quercus acutissima and shrub-grass layer roots were used to determine the biomass and carbon content to establish a regression model of the root biomass and to estimate carbon storage of the underground root and the community. On comparing 11 types of mathematical regression models, the root biomass regression model of Quercus acutissima was constructed, and the root biomass of young forests, half-mature forests, near-mature forests, and mature forests were determined to be 14.81t/hm², 41.15t/hm², 50.36t/hm², and 53.75t/hm², respectively, with the following order of root biomass at each level:root pile > thick root > large root > fine root; shrub and herb root biomass were 0.48-1.71t/hm² and 0.13-0.60t/hm², respectively; the root biomass of different stand age of Quercus acutissima community was 15.42-56.06t/hm², which increased with an increase in forest age. The order of carbon content in the roots of Quercus acutissima was root pile > thick root > large root > fine root, and carbon content was significantly different; the carbon contents of roots of shrubs and herbaceous plants were 41.84%-43.79% and 34.03%-38.48%, respectively, with no significant change in root carbon content of shrub-grass plants with age. Root carbon storage in the tree layer increased with an increase in age, and the root carbon storage of young forests, half-mature forests, near-mature forests, and mature forests was 6.01t/hm², 17.41t/hm², 21.79t/hm², and 21.99t/hm², respectively; the root carbon storage of shrubs and herbaceous plants increased with an increase in forest age; the communities of young forests, half-mature forests, near-mature forests, and mature forests were 6.26t/hm², 17.74t/hm², 22.37t/hm², and 22.94t/hm², respectively, in the order of tree layer > shrub layer > herb layer. Carbon storage and root biomass in the underground biomass of Quercus acutissima forests increased with an increase in forest age. Carbon storage and root biomass in the young forest to the near-mature forest increased rapidly with an increase in forest age, carbon and root biomass accumulation increased gradually after the near-mature forest stage, similar to that in mature forests.