This study aimed to determine the distribution, stability, and soil organic carbon (SOC) of aggregates, and the contribution of soil aggregate proportion, stability index, and aggregate-associated SOC to the total SOC. Three hundred and sixty soil samples were gathered from shelterbelts and neighboring farmlands in five layers of 1 m profiles in Songnen Plain, northeastern China. The shelterbelt plantations were found to increase by 69.5% and 103.8% in >2 mm and 0.25–2 mm soil aggregates, respectively, and their R_0.25, mean weight diameter (MWD), and geometric mean diameter (GMD) were enhanced by 96.3%, 33.2%, and 40.0%, respectively, compared to those of farmlands in soil layers at 0–20 cm depth (p < 0.05). The total SOC content increased by 13.3% at 0–20 cm soil depth, and the SOC content and stock in >2 mm aggregates increased by 21.5% and 18.7% in the 20–40 cm layer (p < 0.05), respectively. The SOC content and stock in total soil had a significantly positive relationship with the proportion of >2 mm soil aggregates and a negative relationship with the value of fractal dimension (D). The enhancement in the SOC of the total soil was dependent on the increase in aggregate-associated SOC, with larger-particle aggregates having a greater contribution. Based on the study results, afforestation improved soil stability and the structure of soil aggregates, and SOC accumulation in the total soil was not only governed by SOC concentration and stock within the aggregate size class, but also the proportion of >2 mm soil aggregates and the value of the fractal dimension.