Abstract: Background Stand density is an important management factor affecting soil structure and functioning in plantations. This study aimed to determine how soil aggregate stability responds to different stand densities in Larix principis-rupprechtii Mayr plantations and to clarify the relationships between aggregate stability and major soil physicochemical properties, thereby providing a scientific basis for stand-density regulation and soil quality improvement in plantation management. Methods The study was conducted in L. principis-rupprechtii plantations at Wangyedian Forest Farm, Inner Mongolia, China. Three stand-density levels were selected: low density (LT, 658±122 trees·hm⁻²), medium density (MT, 1 144±34 trees·hm⁻²), and high density (HT, 1 800±238 trees·hm⁻²). Soil samples were collected from the 0–20 cm layer. Dry sieving and wet sieving were used to determine the size distribution of soil aggregates and to evaluate their mechanical and water stability, respectively. Aggregate stability was characterized using indices including mean weight diameter, geometric mean diameter, percentage of aggregate destruction, and fractal dimension. Major soil physicochemical properties, including soil organic carbon (SOC), total nitrogen (TN), and total phosphorus (TP), were also determined. Analysis of variance was used to examine differences among stand-density treatments. To account for potential topographic heterogeneity among plots, analysis of covariance was further conducted with stand density as the fixed effect and slope as a covariate. Pearson correlation analysis was used to examine the relationships between soil physicochemical properties and aggregate-stability indices. Results (1) Soil aggregate stability was highest in the MT stands. The mean weight diameter of water-stable aggregates were 1.66±0.73mm and 1.55±0.60mm), respectively, representing increases of 45.6% and 39.6% relative to the LTnstands. The aggregate breakdown percentage and fractal dimension were lowest in the MT stands, at 24.89%±4.50% and 3.40±0.20, respectively. (2) Soil particle-size distribution did not differ significantly among the three stand density levels. Soil nutrient contents varied among stand densities. Soil organic carbon (SOC) content was higher in the LT stands (40.39±11.38g·kg-1) than that of MT (26.43±4.69g·kg-1) and HT (29.67±4.48g·kg-1). Howerever, Pearson correlation analysis showed no significant correlation between soil aggregate stability indices and SOC, total nitrogen (TN), or total phosphorus (TP) contents (P>0.05). Conclusions Among the three stand densities examined, the medium-density plantation, approximately 1 144 trees·hm⁻², exhibited relatively high soil aggregate stability. The lack of significant relationships between aggregate stability and SOC, TN, or TP suggests that variation in aggregate stability cannot be adequately explained by bulk nutrient contents alone and may also be regulated by root traits, microbial activity, and organic-matter composition.