Karst landscape in southwestern China is one of the most typical landscapes developed on carbonate bedrock and has the largest area in the world. Carbon sequestration potentials during secondary karst vegetation recovery remain uncertain. Based on the vegetation map and 87 sampling plots at five stages of natural vegetation succession, this study estimated aboveground (AG) vegetation carbon stocks and dynamics at a watershed scale. AG carbon density of grasslands, shrublands, shrub forests, secondary forests and primary forests was 1.70, 4.15, 22.3, 70.3, 142.2 Mg ha⁻¹, respectively. The ten most important species stored 71.6–96.1% of total AG carbon stock, indicating that carbon pool in karst vegetation was determined by a few dominant species. Main contributors to AG carbon stock shifted from individuals in small diameter classes in shrublands to individuals in large diameter classes in primary forests, indicating that carbon increases in the early vegetation succession resulted from high recruitment of woody plants, while carbon accumulations in the later forests were mainly due to tree growth. The long time required for secondary forests to recover carbon density to the level of primary forests could be explained by the slow speed of large evergreen trees reaching a high level of dominance during secondary succession on the harsh habitats. The total AG carbon stock of the studied watershed (7.50 × 10³ ha) was 85.9 × 10³ Mg, of which paddy fields, dry lands, grasslands, shrublands, shrub forests and secondary forests accounted for 22.6%, 3.49%, 0.34%, 5.97%, 12.3% and 55.3%, respectively. The AG carbon stock in this watershed would increase by 92.5% in 50–100 years and by 4.40 times in 140–200 years if the degraded vegetation types could continue to develop into mature forests. Although carbon density of karst forests was significantly lower than that of the forests on non-karst habitats in the same latitudinal zone, the degraded karst vegetation showed a great carbon sequestration potential due to the large distribution area in southwestern China.