Spinel has been widely used in refractories because of its excellent properties, but conventional spinel has the problem of poor activity and high cost. In this study, a highly reactive alumina-rich spinel micro-powder was synthesized at a low temperature by solid-phase reaction using commercial alumina and commercial light-burned magnesia. The MA90 spinel micro-powder was added to MgO-C refractories for the slag line of steelmaking ladles to investigate its effect on the microstructure, physical properties, and corrosion resistance of MgO-C refractories. The results showed that the addition of MA90 spinel micro-powder improved the performance of MgO-C refractories significantly, and its optimal addition was 3 wt%. The addition of MA90 spinel micro-powder optimized the microstructure of the matrix and enhance the mechanical strength and corrosion resistance of the MgO-C refractories. The formation and growth of SiC whiskers in MgO-C refractories facilitated by the addition of MA90 spinel micro-powder.