Processes facilitated by precipitation play an important role on mercury (Hg) accumulation on forest floor and therefore key to Hg cycling in forest ecosystems. Sites along the windward slope of 1,250 to 2,400 m at Mt. Ailao, Southwestern China, have higher precipitation than the leeward slope sites. In this study, measurements of Hg concentration and associated stable isotope composition for soil, fresh, and degraded litterfall samples were made at sites along two slopes of Mt. Ailao to quantify the direct and indirect effects of precipitation on Hg  accumulation on forest floor. Higher soil Hg concentrations, larger litterfall Hg depositions, and faster litter decomposition rates were observed on the windward slope (1,250–2,400 m).Data of Hg isotopic signatures suggest that Hg in surface soils is mainly derived from litterfall Hg input.Precipitation enhances litterfall Hg deposition by increasing litter biomass production, reduces litter decomposition rate, facilitates shortterm Hg uptake to decomposing litter, and potentially increases microbial activity that increases Hg loss via microbial reduction or runoff. Structural equation modeling results support that the indirect effect of precipitation on increased biomass production merge as the most important factor controlling soil Hg variation. Given the climate forcing on global precipitation pattern and vegetation growth cycle, Hg biogeochemical cycling is likely to continue to evolve under the changing climate.