Forest ecosystem is the largest terrestrial ecosystem and plays a critical role in the biogeochemical cycling of mercury (Hg). Litterfall is confirmed to be a dominant pathway for Hg to reach the ground surface under a forest canopy. The objective of this research was to understand the characteristics of Hg migration and transformation in the litterfall and underlying soil during litter decomposition in a subtropical evergreen broad-leaf forest. Therefore, the dynamics of Hg concentrations and speciation distribution in the decomposing litterfall and underlying soils of Simian Mountain Nature Reserve were investigated for 1 year, from March 2014 to March 2015. The results indicated that initial total Hg (THg) concentration in the litterfall was 78.40 ±1.4 (standard error, SE) ng/g, which increased with the decomposition of litterfall. THg content increased up to 101.80±7.6 ng/g after decomposition for 1 year, while THg concentration increased by 30%. THg concentration reached its maximum (120.45 ng/g) after decomposing for 90 days. Compared with THg, the increase of methylmercury (MeHg) in the decomposing litter was more remarkable (P < 0.01). MeHg concentration was 2.38 times that of its initial value (0.32 ng/g) at the end of the decomposition, and its content peaked (0.86 ng/g) after decomposing for 210 days. Generally speaking, the concentrations of both THg and MeHg increased with the decomposition of litterfall. The proportion of inert Hg in the decomposing litterfall was highest at the initial and advanced stages of decomposition, accounting for 75%. Moreover, Hg in the litter was relatively stable at these stages. Therefore, the mobility of Hg was slow, and thus its ecological risk were low. Both the soluble and acid soluble Hg contents increased significantly in the sfpring and summer. At this time, THg and MeHg in the decaying litterfall were inclined to be transported to the downstream water with a large amount of rainwater, thereby causing high ecological risk. On the contrary, the concentrations of THg and MeHg in the organic layer of the underlying soil were quite stable. A comparison of the THg concentrations in the litter and underlying soil revealed that Hg in the soil could enter the litter, so the soil could act as the source of Hg during litterfall decomposition. Results of MeHg content showed that MeHg in the litter could be exported to the underlying soil, and was finally stored in the soil and gradually demethylated. The volatilization of Hg could be one of the reasons why its concentration in the soil was stable. The decomposing litter was also an important source of THg and MeHg to the surface water because it could be transported by the runoff and percolate. The content of organic C in the litterfall decreased by 22%, while the organic N content increased by 15% during decomposition. THg concentration was negatively correlated with the C/N ratio but positively correlated with the N content. The increase of THg concentration was presumed to be closely related with the nitrogen fixation microorganisms. Microbial carbon and nitrogen in the litter increased and were significantly higher in summer than that in winter (P < 0.05). The microorganisms activated in the decomposing litterfalls could immobilize more nutrients as decomposition rates and microbial activities peaked in the summer, and thus contributed to microbial biomass of the soil. Consequently, the concentration of THg increased during this process, especially MeHg, which increased exponentially.