Tooling materials used in the synthesizing of Mg alloys can introduce impurities, which can severely degrade the anticorrosion properties for the final Mg products. To address this issue, this work analyze the purity of molten Mg alloys AZ91D and AM50A in contact with three commonly used steels: 20#, H13, High‐CrNi (022Cr25Ni7Mo4N) for different durations at various temperatures. Interactions between alloying elements and impurity elements are analyzed combining the thermodynamic and kinetics. Results show that low holding temperatures (below 680 °C) can significantly reduce the amount of introduced impurities. H13 steel has the smallest effect on the final purity of the molten Mg alloys and the presence of Cr can inhibit the dissolution of Fe into molten Mg alloys.