Municipal solid waste incineration fly ash (MSWI FA) is globally classified as hazardous waste due to its high levels of complex pollutants and leachable chloride. Typically, chloride in MSWI FA necessitates washing treatment, constituting a significant portion of the total treatment cost. Unlike traditionally removing chloride, this study employs chloride/sulfate in MSWI FA to activate magnesia, generating hydrates for the immobilization of potentially toxic elements (PTEs). Results showed that chloride and sulfate in MSWI FA could form magnesium oxychloride cement (MOC) and magnesium oxysulfate cement (MOSC) hydrates with the aid of crystal nuclei. The MOC and MOSC systems exhibit differentiated bonding mechanisms for typical PTEs (e.g., Pb and Zn) in MSWI FA. The MOSC system demonstrated better immobilization efficiency for Pb than that of the MOC system due to the formation of insoluble PbSO₄ precipitates. In contrast, the MOC system displayed better immobilization performance on Zn than that of the MOSC system due to the superior stability of hydration crystal compounds MgZnCl₄·5H₂O over hydroxide precipitation. Practical application demonstrated that MSWI FA could be solidified using magnesia cement, resulting in satisfactory mechanical strength and efficient PTE immobilization. Overall, this study offered a novel perspective on utilizing components within MSWI FA for enhancing its subsequent stabilization/solidification treatment.