Abstract: In order to explore the improvement effects of polyacrylamide (PAM) and grass roots on soil in badlands, the composition, distribution, structural and fractal characteristics of soil micro-aggregates in purple soil were measured after six months exposure to various treatments, including blank control (CK), polyacrylamide on the soil surface, grass planting alone, “Cynodon dactylon+PAM” and “Trifolium repens+PAM”. The results indicated that the proportion of large particle size micro-aggregates of each experimental treatment increased while the proportion of small particle size micro-aggregates decreased, compared to CK. Hence, the mean weight soil specific area (MWSSA), dispersion coefficient and dispersion rate of experimental treatments significantly decreased (The minimum values were 0.30, 0.15 and 0.60 times those of CK, respectively). The mean weight diameter, aggregation states and degree of aggregation of each experimental treatment significantly increased (The maximum values were 1.17, 1.49 and 1.28 times those of CK, respectively). The fractal dimension of micro-aggregates in experimental treatments also decreased. All the structural parameters of micro-aggregates in soils of treatment GY-PAM₁ (C. dactylon planting and liquid application of 30 g/m³ polyacrylamide) were superior to the other treatments and this treatment showed greater reduction in erodibility. The root system parameters of “grass+PAM” were better than those of grass-planting alone. The total root length and the root length in the diameter (d) class d≤0.2 mm were the major factor and critical diameter class affecting structural stability and erodibility reduction of purple soil in barren hillside badlands.