Al₂O₃ and SiO₂ greatly influence the formation of complex calcium ferrite, which is the main bonding phase in high basicity sinters. The effects of Al₂O₃/SiO₂ ratios on the morphology of complex calcium ferrite were studied. The main mineral phases in the samples with different Al₂O₃/SiO₂ ratios were CaFe₂O₄ with a solid solution of Si or Al atoms and the silico-ferrite of calcium and aluminum. The results showed that the morphology of the complex calcium ferrite changed from lumpy to plate-like and acicular with increases in the SiO₂ content and the Al₂O₃/SiO₂ ratio. When the content of SiO₂ was 4 wt.%, the main calcium ferrite morphology was acicular, and the number of macropores in the samples increased with the Al₂O₃/SiO₂ ratio increasing. The first-principles analysis of the calcium ferrite crystal structure showed that adding SiO₂ and Al₂O₃ changed the growth mechanism of the CaFe₂O₄ crystal, promoting the formation of platy and acicular complex calcium ferrite. The size of calcium ferrite was significantly smaller due to the increase in CaO–Fe₂O₃–SiO₂–Al₂O₃ viscosity with increasing the Al₂O₃/SiO₂ ratio.