Close-to-nature transformation is considered one of the most promising options for creating new forest carbon sinks, but the mechanism by which it influences the biomass by changing the forest structure and thereby influencing the ability and potential of the forest for carbon sequestration remains unclear. Therefore, there is an urgent need to understand these key effects of close-to-nature transformation on the biomass for carbon management in plantation ecosystems. Based on a close-to-nature forest of Pinus massoniana plantation (P(CN)) and an unimproved pure stand of P. massoniana (P(CK)), and a close-to-nature stand of Cunninghamia lanceolata (C(CN)) and an unimproved pure stand of C. lanceolata (C(CK)) as the research objects, the biomass and allocation difference of the four forest types were studied using the method of quadrat sampling combined with biomass measurement, aiming to reveal the influence of close-to-nature transformation on forest biomass and its allocation patterns in P. massoniana and C. lanceolata plantation. The results indicated that the biomass and productivity of P. massoniana and C. lanceolata plantations can be significantly increased by close-to-nature transformation, and the biomass of P. massoniana and C. lanceolata forest stands can be increased by 46.71% and 37.24%, respectively, after 8 years. The biomass of the arborous layer dominates the total biomass (95.48%-98.82%), which plays a vital role in the overall change in forest stand ecosystem biomass. The increase in biomass and productivity in the forest is mainly due to the change in the forest stand community structure, which increases the productivity of the arborous layer. Taken together, the results indicate that reasonable management measures can not only improve stand structure and productivity, but also create favorable conditions that enhance vegetation carbon fixation capacity and potential.