Toxicity Characteristic Leaching Procedure (TCLP, USA) and Acetic Acid Buffer Solution Method (HJ/T 300–2007, China) are often used to simulate the leaching process of heavy metals from MSW incineration fly ash after entering sanitary landfill under the influence of landfill leachate. However, the results based on the standards can neither obtain the actual leaching pattern of heavy metals, nor evaluate or predict their environmental risk evolution. In this study, the leachate characteristics and leachability of Cd, Cu, Pb, Zn, Cr, and Ni during the co-landfilling of chelating agents or phosphate stabilized fly ash and MSW were investigated using three simulated landfill columns under rainfall conditions. The maximum leaching concentration of Pb and Cd in the co-landfill of chelate stabilized fly ash and MSW exceeded the corresponding standard limits, and that of Pb, Cd, and Ni for the scenario of co-landfill of phosphate stabilized fly ash and MSW exceeded the corresponding standard limits. In general, there were higher leaching concentrations of heavy metals in the initial stage (Pb, Cd, Zn, and Cr) and middle stages (Cu and Ni) of the landfill. Compared with those of the landfill with MSW alone, the pH, volatile fatty acids (VFA) and ammonia (NH₄⁺-N) concentration were high in the leachate from the co-landfill with stabilized fly ash and MSW. The pH and chemical oxygen demand (COD) of the leachate might be the primary factors affecting the leaching behavior of heavy metals, whereas VFA and NH₄⁺-N seemed to have little effect. This study demonstrated that evaluating the leaching risk of heavy metals according to actual disposal scenarios, is a more effective method than the standard methods.