[Objective] The offsetting effects of greenhouse gas (GHG) leakage on carbon sequestration and GHG mitigation in the grain for green project (GGP) were analyzed in order to clarify the difference and spatial pattern of actual GHG sequestration and mitigation contribution of different afforestation types to GHG sequestration and mitigation. [Methods] A method for the GHG budget accounting of different types of afforestation (including ecological forest, economic forest, timber forest, and firewood forest) under GGP (i.e., carbon accounting and net mitigation-GGP) was established. Carbon sequestration effects of afforestation were estimated for selected typical tree species in vegetation and soil. Carbon sequestration of post-logging products and mitigation of fossil fuel substitution by biomass energy were determined. GHG leakage caused by afforestation and management was also determined. The net carbon sequestration of the four afforestation types in GGP in Central South and East China was then evaluated. [Results] The carbon sequestration of ecological forest, economic forest, and timber forest, and the mitigation of firewood forest amounted to a total of 648.43 Tg (CO₂) during the period from 2000 to 2019 in Central South and East China. Additionally, the total GHG leakage of GGP reached 96.43 Tg, which could offset 14.87% of the total carbon sequestration and mitigation. The economic forest produced the largest GHG leakage, which reached 57.69 Tg, offsetting 73.95% of its carbon sequestration. The GHG leakage offset 5.75%, 16.59%, and 27.05% of the carbon sequestration or mitigation effect of ecological forest, timber forest, and firewood forest, respectively. The average net carbon sequestration per unit area of GGP in this region was 4.52 t/(ha·yr) (CO₂), ranging from 2.85 t/(ha·yr) in Guangxi to 5.74 t/(ha·yr) in Anhui. The net mitigation rate of GGP was 5.52~8.89 t/(ha·yr) for ecological forest, 0.72~1.56 t/(ha·yr) for economic forest, 0.8~1.61 t/(ha·yr) for timber forest, and 3.18~5.99 t/ha/yr for firewood forest. [Conclusion] GGP in Central South and East China has achieved huge net carbon sequestration and emission mitigation benefits. Ecological forest had an obvious effect on carbon sequestration, and the net mitigation benefit was the most significant. Biomass energy could replace fossil fuel combustion and its mitigation benefit was also very considerable. The influence of fertilizer-related GHG leakage on ecological engineering carbon sequestration cannot be ignored. Fertilizer-related GHG emissions and their offsetting effect on carbon sequestration in the ecological project were nonnegligible because it was the largest GHG emission factor.