Afforestation is regarded as a crucial approach to enhancing terrestrial carbon sinks. Nevertheless, in ecologically fragile regions, the impacts of afforestation on carbon in biomass and soil remain highly uncertain. This study employed field investigations to explore the effects of forestry ecological projects on carbon stocks in biomass and soil within the Qinghai-Tibet Plateau, and to deeply analyze its key influencing factors. The key findings are summarized as follows: (1) The total vegetation carbon stocks of arbor forests and shrub forests (ranging from 7.7 to 24.0 Mg/ha) are 1.3-6.8 times that of grasslands (ranging from 3.5 to 6.1 Mg/ha). Afforestation-induced changes in biomass carbon are primarily attributed to the increase in carbon storage within the arbor-shrub layer, while exhibiting negligible effects on herbaceous layer carbon. (2) The soil organic carbon (SOC) stocks (0-100 cm depth) of forestland, shrubland, and grassland are 39.6-64.5 Mg/ha, 40.7-100.2 Mg/ha, and 43.1-121.9 Mg/ha, respectively. There are no significant differences in SOC stocks among shrubland, forestland, and grassland at either the 10- or 25-year development stage. The SOC stocks of 40-year-old shrubland and forestland are 1.5 and 2.3 times that of grassland, respectively. (3) For 10-year-old and 25-year-old arbor and shrub afforestation, biomass carbon increased while SOC decreased, showing a trade-off. In the case of 40- year-old afforestation, both biomass carbon and SOC increased synergistically. (4) Results from the random forest analysis indicate that the understory herbaceous diversity in this region has a significant impact on biomass carbon sequestration, and that soil total nitrogen, ammonium nitrogen, and nitrate nitrogen determine SOC sequestration. (5) Partial least squares analysis further demonstrates that afforestation promotes the retention of SOC stocks by increasing soil nutrients (especially nitrogen and nitrogen availability). Afforestation in alpine and arid regions, especially 40-year shrub afforestation, holds great carbon sequestration potential. The supplementation of soil nitrogen and phosphorus can enhance the carbon sequestration of this system.