In this work, a microwave-assisted hydrothermal treatment was investigated to solidify the heavy metals of municipal solid waste incineration fly ash in a circulating fluidized bed. The influences of additive dosage, temperature, liquid/solid (L/S) ratio, and reaction time with addition of NaH2PO4 were investigated. The chemical components, hydrothermal product, and the leaching concentration of fly ash were determined by X-ray fluorescence, X-ray diffraction, and inductively coupled plasma atomic emission spectroscopy. Also, pH tests were conducted to assess the environmental adaptability of the treated fly ash. In terms of the solidification effect of heavy metals, the effectiveness of additives was in the order Na2HPO4, NaH2PO4, H3PO4, and FeSO4. Experimental results revealed that heavy metals in fly ash were solidified by a microwave-assisted hydrothermal process, under the optimized conditions of 1.5 mol/kg NaH2PO4, 2 mL/g L/S ratio, 10 min reaction time, and 200 degrees C, aid the heavy metal contents met the standard limitation in GB 16889-2008. In pH tests, it was found that the safety range of the treated fly ash was widened from 7.5-11 to 5-13, which indicated that the properties of environmental stability and acid and alkali-resistance of fly ash were enhanced. Therefore, hydrothermal treatment with microwave heating is a feasible approach for the solidification of heavy metals in fly ash in just 10 min. The treated fly ash is suitable for safe disposal or even for recovery and reutilization.