Objective This study aims to investigate the evolution patterns of soil organic carbon （SOC） in abandoned farmland of karst areas and identify the transition threshold of carbon pool from quantitative expansion to structural stabilization. Methods A typical abandoned farmland of karst areas in Guizhou was selected as the research subject. A space-for-time substitution method was adopted to analyze the differences in SOC content and its fractions， including easily oxidizable organic carbon （EOC）， particulate organic carbon （POC）， light fraction organic carbon （LFOC）， and heavy fraction organic carbon （HFOC）， in the 0~20 cm soil layer across farmland （CK） and succession sequences （5， 10， and 15 a of abandonment）. The carbon pool management index （CPMI） and the HFOC/SOC ratio were used to evaluate carbon pool stability. Results 1） After 15 a of abandonment， SOC content in the 0~10 cm soil layer increased by 70.1% compared with CK， with accumulation mainly occurring in the first 10 a. 2） EOC and POC showed an inverted "U"-shaped pattern with increasing abandonment years and peaked at 10 a， increasing by 91.5% and 107.1%， respectively， in the 0~10 cm soil layer compared with CK. HFOC showed a highly significant continuous linear increase （R²>0.9）， with its contribution remaining at 88%~90%. 3） The 10 a of abandonment was the threshold for carbon pool functional transition from an activity-driven quantity increase stage to a stability-driven quality stabilization stage， at which CPMI peaked and then declined， while the stability ratio increased. 4） Carbon pool remodeling showed a vertical differentiation pattern with earlier transformation in the surface layer and a lagged response in deeper layers. The 0~10 cm soil layer exhibited significantly higher sensitivity to succession and faster stability improvement than the 10~20 cm soil layer. The first axis of redundancy analysis （RDA） explained 57.27% of the variations， and abandonment years were the dominant factor driving changes in carbon fractions. Conclusion 10 a is the key turning point for carbon pool functional remodeling in karst areas， and vegetation restoration promotes the transition from active accumulation to chemical stabilization. This finding emphasizes the importance of focusing on carbon pool transformation in surface soils and enhancing carbon sequestration capacity by utilizing strong mineral protection in calcium-rich habitats.