Coral island is the major terrestrial ecosystem type in South China Sea. Litter decomposition is a key biogeochemical process that drives ecosystem nutrient cycling, but has not been studied in these coral islands. We set up three quadrats in each of the two plant communities dominated by Pisonia grandis and Guettarda speciosa in the Yongxing Island of South China Sea. Decomposition litterbags were deployed in the quadrats in July 2018 to quantify the litter turnover rate and nutrient release pattern. The decomposition experiment lasted for 15 months till October 2019 and we sampled 5 times within the period to weigh remaining litter mass and to analyze the chemical constitutes of the decomposing litter. The analyzed chemical elements included total carbon (TC), total nitrogen (TN), total phosphorus (TP), lignin, cellulose, and semi-cellulose. To examine the role of soil mesofauna with body size of 0.2-2.0 mm in the litter mass loss and nutrient release from decomposing P. grandis and G. speciosa leaf litter, litterbags with two different mesh sizes (0.1 mm and 2.0 mm) were used. The relative mass low difference between these two mesh-size litterbags was used to quantify the contribution of mesofauna. During the earlier 6 months of decomposition, the presence of soil mesofauna increased the leaf litter decomposition rate of P. grandis significantly by 12.3% and that of G. speciose by 4.8% (P<0.05). During the latter 6 to 15 months of decomposition, the effects exerted by soil mesofauna seemed to increase, with decomposition rate being increased significantly by 33.0% and 12.3% for the two litter species (P<0.05). The presence of soil mesofauna accelerated the release rate of total carbon, total nitrogen, cellulose, lignin, hemicellulose from the decomposing litters. Change in soil mesofauna community structure was mainly affected by soil temperature. However, the contribution of soil mesofauna to litter decomposition was not correlated to the changes in mesofauna richness and abundance. We argue that the measured mesofaunas contribution to litter decomposition was most likely affected by the dominant groups such as Acariformes and Parasiformes, but the photodegradation of litter and litterbags of the differed mesh-size might also played a role. The exact contribution of these factors/mechanisms ought to be explored in further studies. Based on the results of this study, we suggest that using P. grandis, in comparison of G. speciosa, as pioneer species in the establishment and restoration of degraded vegetation on the coral islands of South China Sea could better promote the ecosystem nutrient cycling.