The mechanism through which fine root biomass affects soil carbon accumulation after prescribed burning remains unclear. In this study, the biomass of fine roots in different life forms (larch, shrub, and grass) and the total soil carbon (STC) were determined after prescribed burning. Relative to a control, the total soil carbon increased one week after the fire (11.70 mg·g⁻¹; 28.1%) and decreased after 8 months (7.33 mg·g⁻¹; 16.7%), returning to control levels 10 months post-fire. There was a reduction in the larch fine root biomass (FRB) (0.20–0.48 t·ha⁻¹; 35.3%–46.1%; these ranges represent the significant variation interval of fine root biomass after the fire across different time periods, compared to the control) but an increase in the shrub FRB (0.06–0.14 t·ha⁻¹; 101.6%–158.4%) and herb FRB (0.06–0.13 t·ha⁻¹; 591%–3200%) during the vegetation recovery process after the fire. The complementary growth of different plant life forms contributed to the changes in FRB. This also caused changes in the different life forms of FRBs’ effects on STC. Prescribed burning increased the contribution of FRB to STC. The compensatory growth of fine roots from different life forms drives the stabilization of the soil carbon pool after prescribed burning. Prescribed burning reduced the litter fuel and changed the FRB of different life forms, but it did not affect the long-term accumulation of STC.