Abstract: The speciation transformation, influencing factors, as well as bioavailability of mercury (Hg) in soil of the water levelfluctuating zone in the Three Gorges Reservoir Area were simulated. The results showed that Hg in soil under alternative drywet condition could be transformed and released. The total Hg content in the soil was decreased by 28.9% after two “wetdry” cycles. The percentages of the six Hg species (watersoluble, exchangeable, carbonatebound, humicsbound, organicsulf and residual Hg) were 6.1%-16.8%, 5.8%-12.9%, 4.5%-17.7%, 12.5%-29.9%, 5.3%-12.8%, and 34.5%-51.6%, respectively. It was found that Hg in soils was dominantly  residue Hg, whose percentage tended to decrease under alternative drywet condition. The percentage of humicsbound Hg  increased gradually and an increase of the percentage of bioavailable Hg (including watersoluble, exchangeable, carbonatebound, and humicsbound Hg) after two wet-dry cycles were observed. Bioavailable Hg could be easily absorbed by aquatic organisms to enter the food chain, which might increase the ecological risk of Hg in the reservoir.