According to emerging concerns of mercury toxicity and international efforts to reduce mercury applications, the recovery and long-term safe disposal of unused mercury is requested. This study focuses on a scenario that mercury was landfilled as HgS, leached to outer environment, and thus reached to human exposure. Mercury environmental fate model was built to evaluate environmental risk of mercury disposal in landfill sites. In this study, water percolation was modeled as unsaturated flow. Simulation results in the reference scenario indicate non-negligible risk 1400 years after mercury landfill disposal. Mercury concentration in landfill leachate exceeds Japan National Effluent Standards. Long-term monitoring of landfill leachate might be necessary. Parameter sensitivity analysis suggested that rainfall parameters like precipitation period and precipitation intensity could give larger impacts on mercury environmental risk. This suggests that the reduction of rainwater percolation into landfill body is a feasible and effective method to decrease mercury environmental risk.