The failure of municipal solid waste incineration (MSWI) fly ash to meet regulatory standards set through the toxicity characteristic leaching procedure (TCLP) of lead (hereafter, Pb) has resulted in its classification as a hazardous material that is unsuitable for use. In this study, a fluidized bed is proposed as a thermal treatment unit to treat MSWI fly ash. The aim of the present work is to treat fly ash by using a fluidized bed with operating conditions, including temperature, pretreatment, and an additive in the fluidized bed. We also considered the partition of the heavy-metal Pb and the emission of organics (PAHs) in the process. Results indicated that the Pb existed mainly in the solid phase, that is, the fly ash and bed materials, under different conditions. The partitioning of Pb to the fly ash was increased from 42.88 to 68.20%, with the operating temperature increased from 700 to 900 degrees C. When a water-washing pretreatment was applied, the Pb partitioning to the fly ash was less than that when water-washing pretreatment was not applied. This was attributable to the chlorine in raw fly ash that was washed out. Subsequently, the partitioning to the bed materials increased with the addition of CaO to the captured Pb. The concentrations of emitted organics increased when the water-washing pretreatment and additive were used, because of the condensation of organics in fly ash and the lower combustion efficiency. Moreover, the application of the water-washing process resulted in a negative correlation between Pb and the emitted organics during the thermal treatment. However, a positive correlation occurred with the addition of CaO in the fluidized bed.