Based on the investigation data from a total of 15 plots (20 m×50 m) in Cunninghamia lanceolata plantations of different stand ages (6, 16, 23, 32, and 50 years old) in Guangxi of South China, and by using the data of 15 sample trees of different ages and diameter at breast height (DBH), the biomass regression equation with DBH as the single variable was established. The biomass in the tree layer and in the shrub, herb, and litter layers of the plantations was calculated by regression analysis and quadrat harvest method, respectively, and the change trends of the biomass and its allocation in the plantations of different stand ages were analyzed. Except that the stands of 16 and 23 years old had a slight decrease in biomass due to intermediated thinning, the total biomass of the other stands increased with stand age. The biomass of the stands of 6, 16, 23, 32, and 50 years old was 62.73, 172.51, 141.65, 192.30, and 247.32 Mg·hm^-2, respectively, of which, living biomass made up 95.76%-98.39%. Tree layer predominated in biomass, occupying 89.77%-96.55% of the total biomass, and the change trend of tree layer biomass with stand age was as the same as that of the total biomass. The biomass of litter layer was the second, occupying 1.61%-4.24%; while the biomass of shrub and herb layers was the least, accounting for 0.01%-4.26% and 0.27%-4.07%, respectively. The biomass in shrub layer was the largest in 6 years old stand, while that in herb layer was the largest in 23 years old stand. In tree layer, trunk had the greatest proportion of biomass, accounting for 54.89%-75.97%, followed by root, accounting for 11.91%-12.66%. The biomass of both trunk and root was increased with stand age. The proportion of branch and leaf biomass was 11.86-15.19%, and 4.80%-13.17%, respectively, and decreased with stand age. In shrub layer, the biomass allocation was in the order of branch > root > leaf, except for leaf > branch in 50 years old stand, and root > branch in 23 and 50 years old stands. In herb layer, as compared with underground part, aboveground part had greater biomass in 6 and 23 years old stands, but was the opposite in other age stands. The biomass of tree organs and in different layers as well as the whole biomass could be well fitted by growth models. As compared with other plantations, 32 years old C. lanceolata plantation had a mediumhigh level total biomass, suggesting that C. lanceolata would be a fast-growing and high-yielding forestation tree species with high photosynthetic efficiency and high carbon sequestration potential.