The crystallization and heat transfer of CaO–SiO2-based slag for high-Mn–high-Al steel have been investigated by coupling composition evolution with heat transfer and non-isothermal tests. It was found that the concentrations of Al2O3 and MnO in the slag were determined by the relative contents of Al and Mn in molten steel. Crystallization temperature of the slag rose with the increase in Al2O3 content first and then fell due to the coupled effects of basicity and Al2O3, and a small amount of MnO cannot change this trend. High viscosity of slags led to less obvious characterizations of crystallization. The crystal-line phase analysis suggested that MnO accumulation suppressed the formation of fluorite crystals. As the Al2O3 content increased, the precipitation of cuspidine was replaced by the blocky gehlenite crystals, while the number and the size of nepheline and fluorite crystals both increased. According to the thickness and the crystallization structure of slags, the heat flux in the slag film first decreased and then increased with higher Al2O3 accumulation. Then, a new type of CaO–SiO2-based mold slag with high basicity was applied in plant trial, and the change in composition and properties of mold slag was studied.