Currently, most studies related to carbon storage focus on the arbor layer; however, few in-depth studies have analyzed the variations in carbon storage allocation of different layers of plant communities with different stand ages. To estimate carbon density of Pinus tabulaeformis plantation ecosystems for different stand ages more accurately, we tracked 9-, 23-, 33-and 47-year-old P. tabulaeformis plantations in the Hilly Loess Plateau region of China and studied the carbon content in plant organs, litter and soil, as well as the carbon storage and its allocation in different layers. By analyzing carbon sequestration characteristics of P. tabulaeformis plantations at different age stages, this paper provides a theoretical basis of evaluating the function of carbon sinks in P. tabulaeformis plantations during forest management. In addition, the carbon density of the forest ecosystems was measured and evaluated based on "The Research Norms of Chinese Forest Ecosystem Carbon Sequestration, Rate and Potential", which provides a basis for comparative study of cross-regional forest types. The main results were as follows. (1) Average biomass in the different components of the studied forest ecosystems were in the order of: arbor layer (76.12 t/hm²) > litter layer (14.56 t/hm²) > undergrowth vegetation layer (combined herb and shrub layer vegetation; 3.66 t/hm²). The biomass of the arbor layer of P. tabulaeformis increased with stand age. The Pinus arbor organ biomass was as follows: stem accounted for the largest share, followed by leaves and roots, the contribution of branches and bark were minimal. Undergrowth biomass first increased then decreased with increasing tree age, while litterfall biomass increased with tree age. The biomasses of shrub organs were markedly different, which showed that biomass of branches > roots > leaves. In the herb layer, the above-ground portion of biomass was significantly greater than the underground portion. (2) The average carbon content was 50.2% for P. tabulaeformis and for different organs the order was leaves (53.3%) > branches (51.4%) > bark (50.6%) > stems (49.8%) > roots (47.3%). The amount of carbon stored in the shrub, herb and litter layers was 44.5%, 43.8% and 40.6%, respectively. Shrub carbon content of various organs was in the order of branches (46.0%) > leaves (44.8%) > roots (42.5%). The carbon content of herbs was greater in the above-ground portion (45.2%) than in the underground portion (40.2%). Forest age had no significant effect on the carbon content of arbor organs. The carbon content of different shrub organs was significantly different for each herb species. The carbon content of soil (0-100 cm) was between 0.3 and 2.7% and had an obvious vertical distribution characteristic: the surface soil layer had a higher carbon content and carbon content gradually decreased as soil depth increased. (3) Forest age was a dominant factor affecting the carbon density of the P. tabulaeformis forest community. The carbon density of 9-, 23-, 33-and 47-year-old P. tabulaeformis forests was 70.49, 100.94, 167.09 and 144.93 t/hm², respectively. Carbon density in different components of the studied forest ecosystems was in the order of: soil layer > vegetation layer > litter layer. The proportion of vegetation carbon density increased with increasing tree age continually, whereas that of soil carbon density had the opposite pattern. 9-, 23-, 33-and 47-year-old P. tabulaeformis forest arbor layer carbon densities increased with stand age (0.90, 26.56, 59.73 and 60.20 t/hm², respectively), as did carbon density in the litter layer. The vegetation layer and soil layer carbon density first increased and then decreased with increasing stand age.