In order to reveal the biomass distribution and changing law of the tree layer of tropical lowland rainforest, it took the tropical secondary forest of the Maiwan Water Conservancy Hub section of the Nandujiang River as the object of study, and used the biomass estimation regression model to calculate the aboveground growth and increment of the target tree layer, and systematically analyzed the spatial differentiation of the tree layer in terms of its species composition, diameter level, height level, and the gradient of the distance from the river channel. The results showed that: (1) Endospermum and Gironniera were the main contributors of aboveground biomass in the tree layer, with Endospermum ranking first in terms of biomass share (18.17%) and average biomass storage per plant (427.31 kg); (2) the biomass in the tree layer was mainly distributed in the intermediate diameter class group (13 cm≤DBH＜25 cm, accounting for 41.63% of the total) and middle height class group (10 m≤H＜15 m, accounting for 53.93%), but the average above-ground single-plant biomass increment of trees in the extra-large diameter class group (37 cm≤DBH) and upper height class group (20 m≤H) was the largest; (3) the total biomass of the tree layer layer and the average single-plant biomass increment showed a unimodal pattern along the gradient of the distance from the river, with the peaks appearing at 650-950 m from the river channel, the plant community in this area had the widest distribution of diameter at breast height and height class, and also showed the richness of coexistence of trees in multiple diameter and height classes. In addition, the proportion of large diameter class (25 cm≤DBH＜37 cm) and high canopy layer (20 m≤H) trees showed a gradient increasing trend with the increase of the distance from the river channel, and this pattern may be influenced by the fluctuation of groundwater in the riparian zone; (4) the average biomass growth of trees in the first half of the year (March to September of the same year) was generally higher than that of the second half of the year (September to March of the following year). The results of the study preliminarily revealed the distribution of biomass in the arbor layer of tropical lowland rainforests in terms of species composition, diameter class group and height class group, as well as the changes in response to the distance from the river, which will provide a scientific basis for an in-depth understanding of the dynamic process of carbon sequestration in tropical lowland rainforests, biodiversity conservation and ecological restoration.