Municipal solid waste incineration fly ash is classified as hazardous waste because it contains dioxins and a variety of heavy metals. It is not allowed to be directly landfilled without curing pretreatment, but the increasing production of fly ash and scarce land resources has triggered consideration of the rational disposal of fly ash. In this study, solidification treatment and resource utilization were combined, and the detoxified fly ash was used as cement admixture. The effects of thermal treatment in different atmospheres on the physical and chemical properties of fly ash and the effects of fly ash as admixture on cement properties were investigated. The results indicated that the mass of fly ash increased due to the capture of CO2after thermal treatment in CO2atmosphere. When the temperature was 500 °C, the weight gain reached the maximum. After thermal treatment (500 °C + 1 h) in air, CO2, and N2atmospheres, the toxic equivalent quantities of dioxins in fly ash decreased to 17.12 ng TEQ/kg, 0.25 ng TEQ/kg, and 0.14 ng TEQ/kg, and the degradation rates were 69.95%, 99.56%, and 99.75%, respectively. The direct use of fly ash as admixture would increase the water consumption of standard consistency of cement and reduce the fluidity and 28 d strength of mortar. Thermal treatment in three atmospheres could inhibit the negative effect of fly ash, and the inhibition effect of thermal treatment in CO2atmosphere was the best. The fly ash after thermal treatment in CO2atmosphere had the possibility of being used as admixture for resource utilization. Because the dioxins in the fly ash were effectively degraded, the prepared cement did not have the risk of heavy metal leaching, and the performance of the cement also met the requirements.