Global warming is the main phenomenon of current global climate change, which affects the carbon cycle in terrestrial ecosystems. Forest soils are the largest carbon pool in terrestrial ecosystems. Climate warming often affects the accumulation of soil organic carbon (C) and its different fractions in forests. Declines in soil organic C and its active C fractions are often observed under short-term warming, but it is not clear whether this negative effect still persists and whether organic C fractions change under long-term warming. This study investigated the effects of long-term warming on the soil organic C and its fractions in the south subtropical forest by simulating warming with infrared radiation in a monsoonal evergreen broad-leaf forest in Dinghu Mountain, Guangdong province. The observation results of continuous warming from 2017 to 2021 showed that, as compared with the control, warming significantly increased soil organic C by 4.5%, decreased heavy organic C pools by 9.1%, increased light organic C pools by 9.8%, and easily oxidizable C by 5.8% in the surface layer of the soil. However, warming had no significant effect on the levels of microbial biomass C, dissolved organic C, recalcitrant organic C, and complex C. Duration of warming significantly affected soil organic C, microbial biomass C, easily oxidizable C, dissolved organic C, light organic C pools, heavy organic C pools, recalcitrant organic C, and complex C. The interaction between warming treatment and duration of warming significantly affected microbial biomass C, easily oxidizable C and heavy organic C pools, but did not significantly affect soil organic C, dissolved organic C, recalcitrant organic C, complex C and light organic C pools. In conclusion, soil organic C increased due to the increase in active organic C fractions in southern subtropical monsoon forests under long-term warming, and the bioregulatory effect leading the increase in soil organic C may be larger than the protective effect of minerals. However, the decreased inert C fractions and increased active C fractions may lead to a decrease in soil organic C stability. The results of this study explored the response of soil organic C and its fractions to the long-term warming in the surface layer of the southern subtropical forests, in contrast to most studies found that the short-term warming decreased soil organic C in topsoil, and the results may provide scientific basis and theoretical support for predicting the characteristics of the future changes in soil C pools in this region.