Abstract: In this study, the three-stage countercurrent water-washing products of municipal solid waste incineration (MSWI) fly ash were taken as the research objects. Leaching tests were conducted respectively in accordance with three standard methods: HJ 557—2010 Solid Waste Extraction Procedure for Leaching Toxicity Horizontal Vibration Method, HJ/T 299—2007 Solid Waste Extraction Procedure for Leaching Toxicity Sulphuric Acid and Nitric Acid Method, and HJ/T 300—2007 Solid Waste Extraction Procedure for Leaching Toxicity Acetic Acid Buffer Solution Method. The leaching toxicity characteristics of heavy metals in water-washed fly ash under different leaching systems were compared and analyzed, and a simplified overall pollution toxicity index (OPTIs) was introduced to evaluate the environmental risks. The results indicated that the leaching efficiencies of Cu, Zn, Mn, Cd, Pb, and Ni under the acetic acid buffer solution method were significantly higher than those under the other two methods, with concentrations generally 1-2 orders of magnitude higher. Specifically, under the acetic acid method, the average leaching concentrations of Zn and Cd reached 14.00 mg/L and 1.23 mg/L, respectively, exceeding the grade I limits of GB 8978—1996 Integrated Wastewater Discharge Standard by 6.0 and 11.3 times, respectively. In contrast, the concentrations detected via the horizontal vibration and sulfuric-nitric acid methods remained below the regulatory limits. The comprehensive toxicity evaluation identified Cd and Zn as the primary contributors to the leaching toxicity of washed fly ash, driven by the high ecological risk potential of Cd and the high leaching concentration of Zn. Although the current technical specification HJ 1134—2020 Technical Specification for Pollution Control of Fly-ash from Municipal Solid Waste Incineration (for trial implementation) primarily relies on the horizontal vibration method for toxicity evaluation, washed fly ash still poses a risk of heavy metal migration under specific acidic scenarios. Therefore, it is recommended to incorporate the acetic acid buffer solution method as a supplementary evaluation for extreme acidic scenarios in the environmental risk assessment of resource utilization of water-washed fly ash，and to establish a tiered assessment framework, providing a more robust scientific basis for the safe utilization of MSWI fly ash.

Key words: 　washed fly ash, leaching methods, heavy metals, environmental risk