【Objective】 This study aimed to investigate changes in poplar fine-root biomass under different planting densities and spacing configurations and compare any differences. We explored the effects of density regulation and plant spacing on fine-root biomass to understand the response of physiological and ecological processes (especially root effects) in plantations and provide a scientific reference for rational regulation of poplar plantation density and improvement of aboveground productivity.【Method】 Poplar plantations with a stand age of 10 years were selected, and a total of four treatments with two planting densities (low density: 6 m × 6 m and 4.5 m × 8 m; high density: 5 m × 5 m and 3 m × 8 m) and two spacing configurations (square configuration: 6 m × 6 m and 5 m × 5 m; rectangle configuration: 4.5 m × 8 m and 3 m × 8 m) were set up. In total, 12 plots were selected with three replicates for each treatment. Two trees were randomly selected in each plot. Tree roots were sampled at 60 cm from the trunk in two directions using the intact soil-block method. We analyzed differences in total fine-root ( d ≤ 2 mm) biomass, fine-root biomass with different diameter grades (each 0.5 mm increment is a diameter grade), and fine-root biomass at different months under different planting densities and spacing configurations using repeated measures analysis of variance. 【Result】 ① There was no significant difference in total biomass between the high and low planting density stands, but there was a significant difference in the 0-1 mm diameter level in the early stage of the growth season (May-July), with the biomass of low diameter fine roots in the early stage of the growth season being greater in higher density stands than in lower density stands. The effect of spacing configurations on fine-root biomass showed a similar pattern. ② Different spacing configurations led to significant differences in fine-root biomass, which varied with growth stage and stand density. In general, the fine-root biomass in the square configurations was higher than that in the rectangular configurations, except for the fine-root biomass of low density stands in September and November but especially for the low diameter fine-root biomass at the early stage of the growth season (d ≤ 1 mm, May-July). ③ The fine-root distribution in square configurations was relatively even; there were no significant differences in spacing between plants and rows in the square configurations. However, there were significant differences in spacing between plants and rows in the rectangular configurations, but the biomass differences in the 3 m × 8 m and 4.5 m × 8 m plantations showed the opposite pattern. The inter-row differences in the fine-root biomass in the 4.5 m × 8 m forest were greater than the inter-plant differences, whereas the inter-row differences in the 3 m × 8 m forest were lesser than the inter-plant differences. 【Conclusion】 The effects of planting density and spacing configuration reflected neither in the total fine-root biomass nor in the entire growth season. Planting density and spacing configuration only have a significant effect on the biomass of fine roots with small diameter grades, especially in the early stage of the growth season, i.e., from May to July. The square configuration is more conducive to the growth of fine roots. Fine roots more efficiently make use of resources, and to a certain extent, rectangular configurations cause root compression or intense competition for limited resources, as well as wasting of land. In the future, we should further study the relationship between fine-root biomass and stand productivity, fully integrate aboveground and belowground ecosystem processes, fully understand the impact of planting density and spacing configuration on the productivity of plantations, and provide technical support for the improvement of plantation productivity.