The main bonding phase of high basicity sinter is silico-ferrite of calcium and aluminum (SFCA), the chemical composition, microscopic structure and micro-hardness of SFCA have great effect on sinter quality. The acicular SFCA with high iron content is beneficial to improvement of the sinter quality. In this paper, the effect of different MgO/Al₂O₃ ratio on the chemical composition, microscopic structure and micro-hardness of SFCA was studied through cold field emission scanning electron microscope analysis and micro-hardness experiment. The results show that, when the MgO/Al₂O₃ is in the range of 0.46 ~0.83, with the increase of MgO/Al₂O₃ ratio, the micro-hardness of SFCA increases first and then decreases, reaching the maximum value near 0.6. The microscopic structure of SFCA firstly changes from bulk to columnar and needle, and then from needle to block shape, and the distribution is more homogeneous. The content of iron first increases and then decreases, but there is no obvious change rule in the content of other elements. When the content of iron is high, SFCA presents a needle-like structure, and when the iron content is low, SFCA is of flaky, lumpy or plate-like structure.