Abstract: The low-temperature (600-700 ℃) heat treatment of hazardous waste incineration fly ash was carried out using a NaCl-CaCl₂ mixed molten salt system. The effects of heat treatment process parameters (time, temperature, fly ash/molten salt ratio) on the migration and leaching characteristics of heavy metals in the product were studied to reveal the transformation and stabilization mechanism of heavy metals. The results indicated that heavy metals in fly ash underwent direct or indirect chlorination reactions under the action of molten salt or chlorides (NaCl, KCl, etc.) contained in fly ash, and ultimately accumulated in the molten salt phase, while the rest remained in the slag. The proportion of heavy metals in molten salt was significantly correlated with their volatility and heat treatment temperature. After 1 hour of molten salt heat treatment at a temperature of 600 ℃ and a fly ash to molten salt ratio of 1∶5, the proportions of Cu, Zn, Cr, Cd, and Pb in the molten salt were 84.45%, 82.28%, 75.35%, 93.08%, and 96.39%, respectively, resulting in a 63% reduction in the fly ash volume. The leaching rates of Cu and Pb in the sulfuric acid-nitric acid method slag decreased by 88.4% and 97.3%, respectively, compared to the original ash. Zn, Cd, and Cr were not detected in the leachate, meeting the limit values specified in the Standard for Pollution Control on the Hazardous Waste Landfill (GB 18598-2019). The leaching rates of Cr, Zn, Cd, Cu, and Pb in the slag produced by the acetic acid buffer solution method decreased by 90.5%, 99.9%, 99.8%, 98.4%, and 79.6%, respectively, compared to the original ash. The stability of Pb in the slag can be improved by further adding CaO (4%) to the NaCl-CaCl₂ molten salt system.