In order to reveal the effect of different forests on soil aggregate stability in Xiaolongshan forest region of Gansu province, three typical forest stands (Pinus tabuliformis, Larix kaempferi, Quercus aliena var. acuteserrata) were selected as objects of study. The fast wetting (FW) method in Le Bissonnais was used to determine the mean weight diameter (MWD) of soil aggregates in different soil depths (0—10, 10—20, 20—40 cm), and the effects of soil physicochemical properties (soil mechanical composition, SOC, CEC, pH) and root characteristics (proportions of fine root and very fine root, RLD, RMD, SRL) on soil aggregate stability were further studied. The results showed that the stability of soil aggregates under different forest stands were ranked as Quercus aliena var. acuteserrata>Pinus tabuliformis>Larix kaempfer. Stability of soil aggregates of different soil depths were ranked as 0—10 cm>10—20 cm>20—40 cm. According to simple linear regression models, the soil characteristic factors that contributed the most to the MWD of soil aggregates were CEC (R²=0.55) and SOC (R²=0.54), the root characteristic factors that contributed the most to the MWD of soil aggregates were RMD (R²=0.60) and RLD (R²=0.45), and there was a significant positive correlation between soil aggregate stability and the four factors above. Overall, soil aggregate stability differed in different forest stands, soil aggregate stability decreased with the increasing soil depth, and the factor that influenced the stability of soil aggregates the most was RMD.