Litter decomposition plays an important role in the material cycle of forest ecosystems, and the variation of litter diversity directly influences the degree and direction of mixed litter decomposition. This study conducted a one-year mixed decomposition experiment on the leaf litters of seven tree species in subtropical forests, with a total of 624 litter decomposition bags (39 treatments×4 recoveries×4 replicates), to investigate the effects of functional identity, functional diversity, and species diversity of plant communities on the decomposition of the total mass and different carbon fractions of the mixed leaf litter. The results showed that the mixing effects of the leaf litter total mass and different carbon fractions decomposition showed different nonadditive effects. Specifically, 72% of the combinations for the total mass decomposition exhibited synergistic effects, all the combinations of the labile carbon fractions showed antagonistic effects, 56% of the combinations of acid-hydrolyzable carbon fractions showed antagonistic effects, and 97% of the combinations of acid-unhydrolyzable carbon fractions showed synergistic effects. The decomposition of total mass and carbon fractions of mixed leaf litter was predominantly impacted by functional identity, not functional diversity or species diversity. The decomposition of total mass was mainly influenced by the significant negative effect of stoichiometric ratio, while labile carbon fractions were mainly influenced by the significant negative effect of physical properties and the significant positive effect of stoichiometric ratio, and acid-unhydrolyzable carbon fractions were mainly influenced by the significant negative effect of chemical properties. Therefore, distinguishing different carbon fractions of litter to analyze their diversity effects could provide a reference for exploring the mechanism of diversity effects in the decomposition of total litter mass.