In this paper, fly ash and sulfuric acid residue are taken as the research objects, thermodynamic analysis and high-temperature test are adopt to explore the coreduction mechanism of preparing iron-silicon alloy and the influence of calcination temperature on microstructure evolution characteristics and phase transformation law of reduction products prepared by coreduction of fly ash-sulfuric acid residue solid waste system. The results show that there are various phase compositions between iron-silicon alloys, and the main phase of sulfuric acid residue is helpful to promote the reduction of mullite phase and quartz phase in fly ash. XRD analysis shows that the alloy phase in the calcination product is mainly Fe₃Si at the reduction temperature of 1 150 and 1 200 ℃. When the temperature exceeds 1 250 ℃, Fe₃Si phase disappears completely. In the temperature range of 1 300 ~ 1 400 ℃, iron-silicon alloy mainly exists in the form of Fe₃Si₅. SEM-EDS analysis shows that the iron-silicon alloy particles are spherical, and when the reaction temperature is 1 150 ℃, the surface has convex hull containing aluminum, silicon and oxygen with different sizes. When the temperature is 1 250 ℃, the size of convex hull particles on the surface of iron-silicon alloy becomes smaller. When the reaction temperature is increased to 1 350 ℃, the convex hull size on the surface of iron-silicon alloy is smaller and the number is obviously reduced. The research results can provide guidance for the influence of other coreduction factors on the coreduction system of fly ash and sulfuric acid residue, and then lay a foundation for the full resource utilization of the solid waste system of fly ash and sulfuric acid residue.