Abstract: The water washing process is an effective method to reduce chlorine content in municipal solid waste incineration (MSWI) fly ash. However, it generates a large volume of washing wastewater. Utilizing landfill leachate for fly ash washing and subsequently treating the mixed washing-leachate effluent in existing leachate treatment facilities presents a synergistic solution to both waste streams. This study investigated the washing efficiency of various landfill leachates on fly ash under different solid-to-liquid (S/L) ratios. The results showed that at S/L ratios of 1∶3 and 1∶4, the soluble chloride leaching rate of fly ash was lower than 2% for all leachate types tested. Furthermore, the heavy metal leaching concentrations met the requirements specified in the Technical Specification for Pollution Control of Fly-Ash from Municipal Solid Waste Incineration (HJ 1134-2020). Taking the most-abundant metals, Zn and Pb, as examples, their leaching concentrations were both below 1 mg/L, which was comparable to the levels observed when using tap water for washing. Analysis of water quality changes in the leachate before and after washing revealed that, under the S/L ratios of 1∶3 and 1∶4, a maximum COD removal rate of 86% was reached after the first washing cycle Moreover, the concentrations of heavy metals in the wastewater after the third washing cycle were all below 2 mg/L, complying with the relevant pollution control limits set in HJ 1134. Based on these findings, an optimal washing scheme was proposed: The primary washing effluent underwent carbonation for calcium removal. The secondary and tertiary washing effluents were recycled for subsequent washing cycles. The remaining wastewater was treated by membrane filtration and then returned to the reclaimed water system. Experiments conducted with a small-scale treatment setup under this optimal scheme demonstrated significant improvement in the quality of the washing wastewater: Calcium ion concentration decreased from 4 400 mg/L to below the detection limit. Total alkalinity dropped from 4 500 mg/L to 50 mg/L. Chloride content was reduced from 5 743 mg/L to 149 mg/L. COD decreased from 68 mg/L to 34 mg/L. BOD₅ fell from 97.1 mg/L to 7.6 mg/L. Suspended solids (SS) decreased from 10.6 mg/L to 7.42 mg/L. All treated water quality parameters met the requirements for reclaimed water.