【Objective】The restoration of cultivated land to natural grassland can increase soil organic carbon (SOC) content. This study aimed to investigate the changes in black soil organic carbon during vegetation restoration.【Method】Based on a 19-year long-term field experiment, the temporal dynamics of SOC and its fractions were examined during the restoration of cultivated black soil to natural grassland vegetation (GL), with comparisons made to continuous cultivated land (CL) and bare land (BL) without vegetation cover.【Result】The results showed that: (1) Compared to the initial soil, the SOC content in the topsoil (0-20 cm) increased by 26.19% in the GL treatment, with an annual growth rate of 1.38% (0.41 g·kg-1·a-1). In contrast, the SOC content decreased by 7.99% in the BL treatment, while no significant change was observed in the CL treatment; (2) Across the entire 0-100 cm soil profile, GL not only significantly increased the SOC content in the topsoil (0-20 cm), but also increased the SOC content in the subsoil layers (20-60 cm). The increments in the 0-20, 20-40 and 40-60 cm layers were 26.19%, 12.08% and 8.70%, respectively. However, no significant changes in SOC content were observed below 20 cm in the CL and BL treatments; (3) Compared to the initial soil, the GL treatment increased the carbon contents of free light fraction (fLFC), occluded light fraction (oLFC) and heavy fraction (HFC) by 199.45%, 112.83% and 12.00%, respectively. Additionally, GL increased the proportions of fLFC and oLFC while reducing the proportion of HFC in the SOC; (4) For humus fractions, the GL treatment increased the contents of fulvic acid (FA), humic acid (HA) and humin (HM) by 74.82%, 29.69% and 11.46%, respectively, and decreased the HA/FA ratio, indicating a reduction in the humification degree of soil organic matter.【Conclusion】In conclusion, long-term restoration of cultivated land can effectively increase the organic carbon content of black soil and promote the accumulation of labile SOC fractions.