As an essential base for agricultural products in China, the black soil in the Northeast region has an organic matter content of up to ten times that of ordinary soil. However, over-exploitation and utilization in recent years have led to a decline in the quality of black soil. Therefore, it is of strategic significance to study the improvement of soil quality by shelter-belts. In order to explore the effects of shelter-belts on the improvement of black soil quality, the soil under the Populus simonii×P. nibra (P. xiaohei) stands of different ages (Y1: 1-year-old P. xiaohei; Y5: 5-year-old; Y6: 6-year-old; Y8: 8-year-old) was used as the research object in this paper. The differences were analyzed by determining the soil physicochemical properties in the sample plots of P. xiaohei stands of different forest ages, and the under-story soil quality was evaluated by using the minimum data set method. The results showed that: (1) the soil texture of different forest ages was as follows: Y1: sandy soil; Y5: loamy soil; Y6: chalky soil; and Y8: chalky soil. (2) Soil permeability and water retention evolved in a benign direction with forest age, and the top soil bulk density decreased from 1.33 g/cm³ to 1.17 g/cm³, with non-significant differences between different soil layers, and the soil physical indexes had a strong correlation with tree growth and other soil factors. (3) Soil pH decreased with forest age and fluctuated with soil layers, ranging from 6.19 to 6.58. Soil nutrients fluctuated with forest age, with significant vertical variation, except for fast-acting potassium, which was enriched in the surface layer, and the correlation between soil nutrients was weak. (4) The effective state content of soil micro-nutrients varied significantly in different forest ages and soil layers, and the correlation between the elements was strong. (5) Based on the construction of the minimum data set, it was found that the minimum data set indicators affecting the soil quality of P. xiaohei at different forest ages included total nitrogen, total potassium, saturated water holding capacity, quick-acting phosphorus, calcium and manganese, and the minimum data set constructed by the total data set could reflect the effective information on the soil quality of the P. xiaohei forest. (6) The soil quality indices of different forest ages of P. xiaohei ranged from 0.358 to 0.667, and the comprehensive score of soil quality was ranked as Y8 ＞ Y6 ＞ Y5 ＞ Y1. Based on the above findings, P. xiaohei as shelter-belt has obvious effect on the improvement of soil quality in the black soil area, soil water retention and permeability is significantly improved, soil chemical nutrient and trace element content is partially increased, and the recycling effect of nutrients was better with the increase of forest age. In order to realize the rational use of black soil resources, this study suggests to change the under-story set crops to improve the soil quality and crop yield according to the age of shelter-belts, which can provide a scientific basis for the sustainable use of young shelter-belts on cropland in the black soil area.