Abstract: There is mercury in fly ash. The mercury in fly ash is easy to migrate in the environment, which limits the use of fly ash in the field of ecological restoration. Based on the occurrence states and characteristics of mercury, a new idea of removing mercury from fly ash by water leaching was proposed. Single factor experiments were used to optimize the process conditions of this new mercury removal process. And the method of in-situ synthesis of hydrated alumina adsorbent was proposed to purify the waste liquid of mercury removal. The results showed that under optimized process conditions, i.e. the temperature was 75 ℃, the mercury removal time was 2 h and the liquid-solid ratio was 5, the mercury removal rate can reach above 80% and the mercury content in fly ash can be reduced to about 0.1 μg/g. It was far below the risk screening value of soil pollution in GB 15618—2018, which greatly reduced the environmental risk of fly ash ecological utilization. The removal rate of mercury in waste liquid by the in-situ synthesized adsorbent reached about 85%. After purification, the concentration of mercury in the solution was reduced to 2.267 μg/L, which was lower than the limit value of enterprise water pollutant discharge standard.