Stabilization of soil organic carbon (SOC) is critical in reducing atmospheric CO2 concentrations and mitigating climate change. However, the mechanisms of SOC stabilization and accumulation still remain uncertain. Here, the topsoil (0—10 cm) of different plant communities with similar climate condition and same parent material in the northern section of Lianhua mountain in the coast of the South China Sea was selected, Chemometrics, 13C isotopic composition and Mantel test correlation analysis was employed for investigating the effects of biological processes and physico-chemical processes on SOC stability of mountain forest ecosystem in wet and hot monsoon area. The results showed that the topsoil SOC in the study area was relatively stable because the proportion of stable micro-aggregate organic carbon and mineral-associated organic carbon was up to 65.68% in SOC content, and SOC accumulative efficiency was higher (average 0.79). The topsoil SOC pools were predominated by relatively stable functional components of particulate organic carbon (80.74% of SOC). The stability of the topsoil particulate organic carbon was determined by the input of the low quality litters and the high proportion (59.37%) components of the stable micro-aggregate organic carbon. The SOC was significantly positively correlated with Fe2O3 and Al2O3 (P < 0.01), respectively, implying the physico-chemical protection of organic-inorganic mineral complexes formed with Fe oxides and Al oxides was one of the stable mechanisms of SOC in the study area. The soil had still the potential to stabilize and accumulate SOC persistently with proceeding the soil minerals chemical weathering process. It is recommended that the necessity in the future studies for measuring the organic carbon and mineral-associated organic carbon content of riverine suspended material in the "slop-river" system in the rainy season, especially during heavy rainfall, to accurately evaluate the SOC sequestration change of forest in hot and humid areas in China. These results can provide basic data for the study of soil C sink mechanism in hot and humid areas and the realization of carbon neutrality in China.