Understory, an important component of the forest ecosystem, plays an important role in maintaining forest structure and soil quality. To analyze the relationship between soil dissolved organic carbon (DOC), belowground biomass, and DOC leachates of aboveground biomass, we conducted a comparative experiment of the presence or absence of understory Dicranopteris dichotoma on severely eroded red soil. The results showed that the aboveground and belowground biomass, and the content of soil DOC increased significantly with understory D. dichotoma coverage (P < 0.05); the effect of D. dichotoma on the surface soil (0-20cm) was greater than the deep soil (20-100cm) (P < 0.05). The correlation analysis showed that there was a significant positive correlation between soil DOC storage and fine root biomass (P < 0.05), indicating that the vertical distribution of underground roots directly affects the soil DOC reserves. Moreover, there was a significant positive correlation between soil DOC with fresh leaf (Pinus massoniana and D. dichotoma) and litter (P. massoniana and D. dichotoma) leachates during vegetation restoration (P < 0.01), but there was only a significant correlation with fresh leaf (P. massoniana) leachates in the bare ground (P < 0.05). The effect of the litter leachates on the soil DOC storage was higher than the fresh leaves. The soil microbial biomass carbon and microbial entropy in D. dichotoma-covered land were significantly higher than bare land in the process of vegetation restoration. Therefore, D. dichotoma may provide more substrates to participate in soil material and nutrient cycling, contributing greatly to soil DOC, which further provides important nutrient reabsorption sources for restoration of P. massoniana forest in eroded areas. It is possible that D. dichotoma coverage increases microbial activity and promotes assimilation of soil microorganisms; thereby playing an important role in the accumulation of soil organic carbon by improving utilization of microbial carbon sources.