Cunninghamia lanceolata plantation with densities of 1800, 3000 and 4500 plants/hm² were employed to investigate the carbon density and its distribution characteristics of C. lanceolata forest ecosystem under different stand densities through field investigation, sample collection and analysis. The results showed that: 1) the carbon density in the C.lanceolata ecosystem with stand densities of 1800, 3000 and 4500 plants/hm² were 131.54 t/hm², 161.42 t /hm² and 172.69 t/hm², respectively, and increased significantly with the increase of stand densities (P < 0.05). The carbon density of C. lanceolata forest followed the order of soil layer > tree layer > understory layer. Soil organic carbon storage accounted for the largest proportion of total carbon storage (53.11%-67.37%), followed by trunk, root and bark (25.89%-35.74%). High stand density could increase the carbon density distribution portion in trunk, bark and root. 2) The carbon density of tree layer, trunk, bark and persistent withered branches and leaves increased with the increase of stand densities, while the carbon density of fresh branches and leaves displayed first increase and then decrease tendency with the increasing stand densities, which all showed significantly decreased with the increase of stand densities (P < 0.05). The carbon density of trunk and bark decreased with the increase of tree height. The carbon density of fresh branches and leaves were relatively high in the middle and upper part of tree (8 m≤h≤10 m), while for withered branches and leaves, higher carbon density was observed in the middle of the tree (4 m≤h≤8 m). 3) The carbon density in roots of different diameter increased significantly with the increase of stand densities (P < 0.05). The carbon density among different roots diameter was as follows: root head > coarse root > big root > middle root > small root > fine root, and root head and coarse root accounted for the largest portion (57.38%-70.84%). 4) The carbon density of undergrowth vegetation decreased with the increase of stand densities, while the opposite trend was observed for litter, which showed significantly decreased with the increase of stand densities (P < 0.05). 5) The carbon density of soil layer decreased with the increase of soil depth, and no significant difference was observed for different stand densities (P>0.05). 6) In summary, appropriate increase stand density is beneficial to improve the carbon density of C. lanceolata forest ecosystem.