Dioxin-like pollutants in municipal solid waste incineration fly ash (MSWIFA) can pose serious health and environmental risks, and it is necessary to understand the degradation mechanism of dioxins in the solid phase to help them degrade to environmentally acceptable levels. This study combined theoretical and experimental approaches to investigate the degradation mechanism of dioxins in MSWIFA with a low-temperature thermal treatment. Octachlorodibenzo-p-dioxin (OCDD) was used as a target contaminant that was attached to simulated fly ash (FA, mCaO
=3:1), and the low-temperature thermal treatment degradation mechanism of OCDD in the solid phase was analyzed with density functional theory (DFT) and to conduct simulated FA degradation experiments. The simulated FA prior to degradation was initially analyzed by thermogravimetric (TG) analysis, and then the solid products were characterized by Fourier-transform infrared (FTIR) spectroscopy and transmission electron microscopy (TEM). The reaction gas was collected and tested to verify the reaction mechanism. The results showed that: (1) there were three stages of weight loss in the simulated FA from room temperature to 400 °C, and the weight loss rate was 9.61%. (2) -OH was the key functional group for the degradation of OCDD by low-temperature thermal treatment. (3) Combining the FTIR and TEM analysis results, it can be seen that when multiple -OH groups replace the Cl on OCDD, they oxidize OCDD to generate a large amount of CO32−
, and this is the primary pathway for OCDD conversion. A sub-pathway of OCDD conversion is the formation of a small fraction of C-O- structures during degradation, which may combine with other degraded OCDD molecules to form macromolecular organics. (4) Combined with the gas-phase test results, it is clear that the primary pathway of Cl conversion is the complete substitution of Cl by the whole -OH group and the substituted Cl will leave the reaction system to form Cl2
. The sub-pathway is that only -O- on -OH replaces the Cl to form C-O-, a structure that is unstable because of the incomplete pairing of radicals, and the Cl and H combine to form HCl to leave the reaction system. The average CO2
content was only 0.0085%, thus indicating that almost no carbonate decomposition or reaction with HCl took place. The study reveals the key groups of dioxins degradation by low-temperature thermal treatment and the material fate of dioxins and Cl, providing theoretical guidance for industrial optimization and parameter setting of low-temperature thermal treatment.