In this study, the migration characteristics and environmental risk of heavy metals (HMs) during the thermal dechlorination process of fly ash (FA) are investigated. The influence of dechlorination temperature, steam addition (mass ratio of steam to fly ash), residence time, and Fe-0/Al-0 additives on the volatilization characteristics, leaching behavior, and chemical speciation of heavy metals is studied. The results indicated that the volatilization of HMs increased with a rise in dechlorination temperature. After thermal processing, the leaching concentration of all HMs, except for Cr, decreased, and the stable fraction increased. Steam addition and an extension of the residence time had a positive action in the volatilization of HMs, as well as inhibiting leaching. The addition of Fe and Al showed little effect on the total volatilization of HMs, and the leachability was well-controlled. A chemical speciation analysis of dechlorinated fly ash (DFA) with Fe-0/Al-0 added revealed that all the HMs were present in the stable fractions, which were oxidizable and residual fractions. Moreover, the risk assessment code and individual contamination factor results suggested that Cd and Zn in raw fly ash resulted in a high environmental risk classification. The risk of all the HMs reduced by almost one grade through the dechlorination process; specifically, the risk grade classification was low risk or no risk.