The behavior of acidity neutralization capacity (ANC) depletion and change of pH values in leachate from two representative samples were investigated with test of general acidity neutralization capacity (GANC) and multistage extraction test (MET). The results indicated that low estimation of potential environmental risk would be conduced with low depletion of ANC and high pH values of leachate in such actual batch test. Comparing the results of GANC with that of MET, neutralization and wash-out process would take place at the same way under the flow-through scenarios of co-disposal, and there is one-third alkaline components reacting in the neutralization process. With simulative calculation basing on reasonable hypothesis of parameter in sanitary landfill, the extraction procedure for leaching toxicity of fly ash from MSWI under codisposal scenario was proposed, using leachant of 0.3 mol x L(-1) HAc solution and L/S =20. This procedure would definitely provide disposal of MSWI fly ash in China with reliable safeguard and effective management.