【Objective】 The study aimed to investigate the response of the fine roots of the 2-year-old ‘Beilin 1’ cultivar of young Populus tomentosa to variations in the species composition of trees and stand density, and to study their distribution and adaptation strategies following variations in species composition and stand density at the young stand stage. The initial plantation and the mixed configuration were used as references.【Method】A mixed P. tomentosa forest (plant spacing 3 m × 6 m) and pure biennial P. tomentosa forests of three different densities (3 m × 3 m, 3 m × 6 m, and 6 m × 6 m) were selected as the research objects. The root drilling method was used for sampling, and the samples were collected from a depth of 150 cm in the vertical direction, with every 10 cm representing a different layer. The sampling points were located at distances of 10, 30, 50, 100, 150, 200, 250 and 300 cm from the trees in the horizontal direction, while the sampling points for the 3 m × 3 m stand were located at distances of 10, 30, 50, 100 and 150 cm, a total of 1 305 root samples were obtained. The obtained root samples were scanned, dried, and weighed for measuring the dry mass. The morphological characteristics of the fine roots and the biomass data at various depths and horizontal distances were assessed across different species compositions and stand densities. 【Result】The findings revealed that the species composition did not significantly affect the fine root biomass density, but significantly affected the root length density and root surface area density. The characteristics of the fine roots of P. tomentosa in the pure and mixed forests did not exhibit any alterations in the vertical direction, all decreased with an increase in soil depth. The fine root biomass density of P. tomentosa in the mixed and pure forests was primarily concentrated at distances of 0-50 and 0-10 cm, respectively, from the trees in the horizontal direction. The biomass density of fine roots of P. tomentosa in mixed forests was mainly concentrated at 0-50 cm. Analysis of the two-dimensional distribution revealed that the fine roots of P. tomentosa were more densely distributed at greater depths in the mixed forest, their horizontal distribution was relatively uniform, and the horizontal lateral roots were more elongated. The stand density did not significantly affect the fine root biomass density, and the root length density, root surface area density, and average diameter of the roots in the high-density stands were significantly greater than those of the other two stands with different densities. The fine root biomass density in the 40-50 cm soil layer of the 3 m × 3 m stand was significantly higher than that of the other two stands (P < 0.05). The finer roots were more densely distributed in the deeper soil layers. The fine root biomass density in the 3 m × 6 m and 6 m × 6 m stands decreased gradually at increasing soil depths, while that of the 3 m × 3 m stand gradually increased in the 0-60 cm soil layer, indicating a unimodal distribution pattern. The root length density and root surface area density of the 3 m × 3 m stand at 150 cm from the trees in the horizontal direction were significantly higher than those of the other two stands of different densities. Analysis of the two-dimensional distribution pattern revealed that the fine roots in the 3 m × 3 m stand were more densely distributed at greater depths, while those in the 3 m × 6 m stand were more horizontally elongated.【Conclusion】Mixing significantly reduced the root density and root surface area density of P. tomentosa. An increase in stand density caused the fine roots to be gradually elongated with depth, and increased their distribution in the deeper soil layers. The fine root biomass density gradually decreased in the horizontal direction at increasing distances from the trees in the high-density stand. The species composition and stand density did not significantly alter the fine root biomass density of young 2-year-old P. tomentosa trees, but the root length density and root surface area density in the pure forest and the 3 m × 3 m stand were significantly higher than those of the mixed forest and the other two stands with different densities. The growth of the fine roots of P. tomentosa was most vigorous in the pure forest and in the 3 m × 3 m high-density stand. The soil water and nutrients were fully utilized by morphological plasticity rather than by alterations in biomass, and the soil resources were more effectively utilized in the young forest than in the mixed forest and in the forest stands of varying densities.