“Grain for Green” Project is one of the main measures to control soil and water loss on the Loess Plateau. Discussing the impact of returning farmlands to forests on the stability of soil aggregates and soil erodibility could provide a scientific basis for ecological restoration and benefit evaluation of soil and water conservation on the Loess Plateau. In this study, the Robinia pseudoacacia L. forests with different planting years (5 years, 10 years, 15 years, 20 years, 25 years and 30 years) were selected as the research object, and the farmland was used as the control to study the dynamic changes of soil aggregate stability and soil erodibility in 0-30 cm soil layers after returning farmlands to forests, and the study also explored the relationship between soil erodibility and soil aggregate stability. The results showed that: (1) The content, mean weight diameter and geometric mean diameter (GMD) of water-stable aggregates＞0.25 mm increased with the increasing of planting years, which increased by 32%~79%, 32%~98% and 2%~60%, respectively, compared with that of farmland. (2) The fractal dimension of soil aggregates decreased with the increasing of restoration ages, which decreased by 0.6%~6.0% compared with that of before returning farmland. Soil organic matter increased with the increasing of restoration ages, increased by 8.4%~38.9% compared with that of farmlands. (3) Soil erodibility factor K increased with the increasing of soil layer, but decreased gradually with the increasing of restoration ages, and decreased by 1.0%, 2.7%, 3.6%, 3.9%, 5.0% and 7.9%,respectively, with the increasing of restoration ages. (4) After the conversion of farmland to forest, the fractal dimension of soil aggregates and the content of water-stable aggregates＞0.25 mm were the main driving factors for the change of soil erodibility. The aboveground biomass indirectly affected soil erodibility through affecting the fractal dimension of soil aggregates and the content of water-stable aggregates＞0.25 mm, and the total effect was the largest. The study suggested that the increase of soil aboveground biomass after returning farmlands to forests played an important role in the formation and stability of soil aggregates and the reduction of soil erodibility, and returning farmlands to forests significantly improved the stability of aggregates and reduced soil erodibility.