【Objective】We focused on the relationship between the microbial diversity and nutrient composition of Casuarina equisetifolia litter by determining changes in the pattern of litter nutrient decomposition and the characteristics of microbial carbon source utilization.【Method】 As a study site, we selected a C. equisetifolia plantation along the coast of the Guilinyang area of Haikou City in Hainan Province, China, which comprises trees of between 10 and 15 years of age. Using the S-type five-point sampling method, we obtained samples representative of three stages of litter decomposition from soil depths of 0-5 cm, ≥5-10 cm and ≥10-15 cm, respectively. The Biolog microporosity plate method was used to determine the functional variation in the diversity of the surface microbes and endophytes of litters at the different stages of decomposition. Principal component analysis (PCA) and redundancy analysis (RDA) analyses were used to determine differences in the utilization of different carbon sources. 【Result】 ① With an increase in the degree of litter decomposition, we observed decreases in the contents of organic carbon, total nitrogen and total phosphorus. ② We also detected significant differences in the functional diversities of exogenous and endophytic bacterial communities at the different stages of decomposition. The average well color development(AWCD) of exogenous microbes increased with an increase in the degree of decomposition, whereas that of endophytes decreased. ③ The Shannon and Simpson diversity index of surface microbes were obviously increased with the increasing decomposition degree of litters. However, there were no significant differences in McIntosh indexes. The Shannon and Simpson diversity index of endophytes of litters showed a similar trend with surface microbes, but the McIntosh index significantly decreased. ④ Exogenous and endophytic bacteria showed different efficiencies in the utilization of six major categories of carbon sources. For both surface microbes and endophytes, the primary carbon sources utilized during the early stages of decomposition were carbohydrates, followed by amides and phenolic acids in the latter stages. ⑤ The PCA results revealed significant differences in litter microbial function among the different degrees of decomposition, and RDA indicated that organic matter and total nitrogen can significantly affect the diversity of surface microbes. 【Conclusion】 Our results revealed that carbon metabolic activity and surface microbe diversity increased with an increase in the degree of litter decomposition, whereas endophyte metabolic activity decreased in response to an increase in microbial diversity. Moreover, we found that pH and total nitrogen content were the two critical factors determining microbial diversity, and that the surface microbes and endophytes associated with litter play different roles during the different stages of decomposition.