Soil light fraction organic matter is an important component of soil organic matter. It is crucial to understand the relationship between soil organic matter and climate change by studying the variation of light fraction organic matter along an elevation gradient in subtropical China. Taking evergreen broad-leaved forest (EBF), mixed coniferous and broad-leaved forest (BCF), and coniferous forest (CF) at different altitudes (from low to high elevation) in Wuyishan National Park in Fujian Province as research objects, the light fraction organic matter in surface soil (0-5 cm and 5-10 cm) was separated by density fractionation analyses, as well as the characteristics of light fraction organic matter and its influencing factors in different forest ecosystems were studied. The results showed that the content of light fraction organic matter in BCF was higher than that in CF and EBF in 0-5 cm and 5-10 cm soil layers (P < 0.05). The pattern of the content of light fraction organic carbon was similar to the light fraction organic matter, but there was no significant difference in light organic nitrogen content (P > 0.05). The light fraction C/N was greater than soil C/N, and the light fraction C/N and soil C/N in BCF were greater than those of other forest types in the same soil layer. In 0-5 cm and 5-10 cm soil layer, the storage of light fraction organic carbon and nitrogen in BCF were higher than those in CF and EBF (P < 0.05), and the contribution of light fraction organic carbon and nitrogen to soil organic matter in BCF was higher than that of the other two forests. The contents and storage of soil organic carbon and total nitrogen in CF was the highest in 0-10 cm, and decreased with the decrease of altitude, but the difference was not significant (P > 0.05). The correlation analysis showed that the storage of light fraction organic carbon and nitrogen was significantly correlated with soil organic carbon, dissolved organic carbon, microbial biomass carbon and fine root biomass (P < 0.05), but not annual litterfall (P > 0.05), suggesting that root derived organic matter might be an important source of light fraction organic matter in soil. Therefore, we concluded that belowground sources may play an important role in regulation of the formation of light fraction organic matter in soil under future climate change.