Large areas of P. massoniana plantations are often assumed to have serious ecological problems such as reduced biodiversity and soil degradation. Logging gaps are an efficient practice used to solve these problems. We investigated how the size of forest gaps in P. massoniana plantations influenced the characteristics of different soil labile organic carbon fractions and the carbon management index. Compared with those in P. massoniana plantations, the content and proportion of microbial biomass carbon increased, while the content of water-soluble organic carbon and readily oxidizable carbon, and the proportion of water-soluble organic carbon decreased in forest gaps. The size of the forest gap had significant effects on the content and proportion of labile organic carbon fractions. With the increasing size of forest gaps, the content of water-soluble organic carbon, microbial biomass carbon, and readily oxidizable carbon increased. The proportions of water-soluble organic carbon and microbial biomass carbon increased, the proportion of readily oxidizable carbon first decreased and then increased and the non-labile carbon proportion first increased and then decreased. The content and proportion of labile organic carbon and the stability of organic carbon were relatively higher in larger (900-1225 m²) gaps. The carbon management index was not significantly affected by forest gap size, which positively correlated with the content of different soil labile organic carbon fractions and soil total organic carbon. The effects of forest gap size on soil organic carbon were relatively significant in larger (900-1225 m²) forest gaps in terms of a higher stability of organic carbon and a greater amount of microbial biomass carbon and water-soluble organic carbon. This suggests that 900-1225 m² gaps might provide optimal logging gap size for the conservation of ecological functions and natural regeneration in P. massoniana plantations.