[Objective] To explore the effects of poplar farmland shelter belts on the structure of soil under forest in order to provide guidance for rational allocation of tree planting density in black soil region.[Methods] This study investigated soil aggregate and analyzed the aggregates contents, percentage of aggregate disruption, mean weight diameter, geometric mean diameter and fractal dimension stability based on different space(1.5 m×1 m, 2 m×1.5 m, 3 m×1.5 m) in Baiquan County, Heilongjiang Province. Abandoned farmland was taken as control treatment.[Results] ① The percentage of aggregate disruption(PAD) of surface soil was the lowest and the percentage of aggregate disruption decreased with the decrease of forest belt density. The percentage of aggregate disruption in forest belt was less than control. ② With the increase of soil depths, mean weight diameter(MWD) and geometric mean diameter(GMD) decreased. There was a significant difference between forest belt and control(p<0.01). ③ At 0-20 cm depth, the dry-aggregates fractal dimension(D) and water-stable aggregates fractal dimension(D) decreased with the decrease of forest density, while an opposite trend was showed at 20-60 cm depth.[Conclusion] The structure of soil under the forest belt is better than the abandoned farmland. The forest belt increases soil clay content and enhances the stability of the deep soil. As the increase of forest belt density, the soil under forest will be improved effectively, but the aggregate disruption in surface soil will be increased.