In this work, the leaching experiments indicated that municipal solid waste incineration (MSWI) fly ash, stabilized with chelating agent or phosphoric acid and solidified with Portland cement, can effectively decrease the leaching of most heavy metals. However, the leachability of some heavy metals will be promoted under low end-point pH conditions. The distilled water leaching decreased the proportion of bioavailable (exchangeable and carbonate-bound) fractions in residual fly ash solids, though the bioavailable fractions will re-increase after leaching under extremely acidic conditions. After exposing the raw or solidified/stabilized fly ash to increasing acidic conditions, not only the highcontent Pb/Zn/Cu, but also some low-content Cd may pose potential risks to the environment. Therefore, in actual disposal situation, the solidified/stabilized fly ash, which is initially classified as posing a low risk level before landfill disposal, may transform with time after exposure to increasingly acidic conditions, such as increased proton activity caused by landfill microbes, atmospheric CO2, acid rain, and/or severe long-term soaking/leaching of acidic leachate in MSW landfills, and hence, pose high or very high levels of risk. Additionally, future studies should focus on further leaching experiments considering the real-world landfill environment, which can help to further study the leaching behavior and environmental risk of heavy metals in MSWI fly ash under complex landfill conditions.