The aims of this research were to (i) systematically investigate the soil organic carbon (SOC) and labile SOC fraction dynamics over a period of 3 years under biochar amendment, (ii) reveal the relations of labile SOC fractions to SOC, and (iii) evaluate the sensitivity of SOC to biochar added at different rates by determining C pool management index (CPMI). The SOC, labile SOC fractions, and the CPMI in the 0–20-cm layer were analyzed via a 3-year field experiment of maize. Four biochar treatments were studied, with application rates of 0, 15.75, 31.5, and 47.25 t ha−1 (CK, BC1, BC2, and BC3, respectively). Biochar was applied manually before sowing only in the first year of this experiment; an equal mineral NPK fertilizer was applied to each treatment annually. The average data of this 3-year field study demonstrated that biochar incorporation significantly increased SOC, particulate organic carbon (POC), easily oxidizable carbon (EOC), light fraction organic carbon (LFOC), and microbial biomass carbon (MBC) by 31.75–83.62, 92.72–323.30, 29.90–51.55, 194.30–437.37, and 31.13–93.12%, respectively, compared to the control; their concentrations increased with increasing biochar addition rates, except for MBC. In addition, EOC, POC, and LFOC were significantly positively related with SOC. Compared to the control, the DOC contents were reduced after biochar addition, but the specific reasons for this finding need to be further studied. Biochar incorporation could not only significantly improve the soil quality via increasing the soil organic C fractions, but also increase C sequestration rates in the long term by increasing the non-labile C pool (NLC). The CPMI could be used as a representative index in evaluating the impacts of biochar on SOC content and soil quality.