Revegetation is an effective means to increase the soil organic carbon (SOC) stock and reverse SOC loss worldwide. Restoration through grasslands has great potential primarily due to the contribution of the root system. However, the impacts of grassland restoration remain uncertain, as current understandings of the direction and magnitude of changes in different SOC fractions and the influence of root properties on SOC are still unclear. To address this problem, we conducted a field study to investigate variations in SOC fractions and root properties under different grassland restoration times (1, 5, 10, 15, 25 and 30 years following restoration) in the Loess Plateau, China. Specifically, we measured fractions of soil labile organic carbon including microbial biomass carbon (MBC) and dissolved organic carbon (DOC) and fractions of soil stable organic C including humus carbon (HS), fulvic acid carbon (FA) and humic acid carbon (HA) in the 1 m soil profile. Furthermore, we evaluated root biomass, root C, root lignin and cellulose as potential drivers of SOC patterns. We found that SOC, soil labile organic C and soil stable organic C fractions generally increased with the revegetation time, except for brief decreases at the 5- and 15-year marks. Across the entire period (year 1 to 30), the proportions of HS and FA to SOC increased by 10.64% and 14.72% at 30 years, respectively, while the ratios of MBC and DOC to SOC were relatively stable. The average SOC accumulation rate was 0.72 Mg ha⁻¹ yr⁻¹ in the 1 m soil profile over the 30-year grassland revegetation period. The average rate of change of FA (0.67 Mg ha⁻¹ yr⁻¹) was higher than that of the HA, HS and soil labile organic C fractions, which indicated that SOC stability increased and SOC accumulation was mainly derived from the enhancement of FA. Decreases in root biomass and cellulose storage drove reductions in SOC fraction stores with soil depth. The root cellulose exerted significant influences on the SOC and SOC fractions, especially on FA (R² = 0.82, p < 0.01). These findings show that grassland restoration plays an important role in improving and increasing SOC; in particular, the increase of FA results from the augmentations of root cellulose.