Phosphorus is considered a key factor of limiting the forests growth in subtropical areas. Understanding the effects of soil microbial functional diversity on soil phosphorus is important for sustainable management of plantations in subtropical areas. Three C. lanceolata + P. bournei (4a,7a,11a) multi-layered mixed plantations were selected in Guanshan Forest Farm, Jiangxi Province. The contents of total phosphorus, available phosphorus, and inorganic phosphorus were determined in these stands. Biology-ECO method was used to study the characteristics of carbon utilization by soil microorganisms in the topsoil layer (0-20 cm) of C. lanceolata + P. bournei multi-layered mixed plantations. The relationship between soil phosphorus fractions and soil microbial functional diversity was analyzed. The results showed that: (1) the contents of soil total phosphorus, available phosphorus, and inorganic phosphorus gradually increased with the increasing age of multi-layered mixed plantations. (2) the functional diversity of microbial communities showed great variations in different stands. Carbon metabolic activity and diversity index also increased with the increasing age of multi-layered mixed plantations. Major carbon sources which were utilized by soil microbes in C. lanceolata monoculture plantation were polymers. They were carbohydrates, carboxylic acids and phenolic acids in 7-year-old multi-layered mixed plantations, while more amino acids, amines, polymers, carboxylic acids and phenolic acids in 11-year-old multi-layered mixed plantations. In 11-year-old multi-layered mixed plantations, the utilization intensities of carbohydrates, amino acids, carboxylic acids, amines and phenolic acids were significantly higher than those in 4-year-old multi-layered mixed plantations and C. lanceolata monoculture plantation. The characteristics of carbon utilization of soil microbial communities showed little difference between 4-year-old multi-layered mixed plantations and C. lanceolata monoculture plantation (except polymers). (3) soil microbial diversity indexes, amino acids, amines and phenolic acids were significantly and positively correlated with soil total phosphorus, available phosphorus, Al-P and Fe-P content. Based on random forest model analysis, amino acids, amines and phenolic acids were the main carbon sources for soil microbial utilization in different stands. In summary, C. lanceolata monoculture plantations converted to multi-layered mixed plantations will be beneficial to the storage and supply of soil phosphorus. The metabolic functional diversity of soil microbial community may play a key role in regulating soil phosphorus availability in multi-layered mixed plantations.