The metallurgical quality control of magnesium (Mg) and Mg alloys in melting process is required to ensure a satisfied mechanical and corrosion performance, while the typical used steel crucible introduces impurities and interfacial interaction during melting process. Therefore, a systematic study about impurities diffusion and interfacial interaction between molten Mg and steel is necessary. In the present study, the interfacial reaction between molten AZ91D Mg alloy and mild steel during melting process was investigated with the melting temperatures of 700 °C, 750 °C and 800 °C. The results show that Al(Fe, Mn) intermetallic layer is the intermetallic primarily formed at the interfaces of AZ91D melt and mild steel. Meanwhile, Al8(Mn, Fe)5 is indexed between Al(Fe, Mn) and AZ91D. AlFe3C appears between the mild steel and Al(Fe, Mn) at 700 °C and 750 °C, but absent at 800 °C due to the increased solubility of carbon in Mg matrix. It is found that the growth of the intermetallic layer is controlled by diffusion mechanism, and Al and Mn are the dominant diffusing species in the whole interfacial reaction process. By measuring the thickness of different layers, the growth constant was calculated. It increases from 1.89(±0.03)×10-12 m2 ·s-1 at 700 °C to 3.05(±0.05)×10-12 m2 ·s-1 at 750 °C, and 5.18(±0.05)×10-12 m2 ·s-1 at 800 °C. Meanwhile, the content of Fe is linearly increased in AZ91D with the increase of holding time at 700 °C and 750 °C, while it shows a significantly increment after holding for 8 h at 800 °C, indicating holding temperature is more crucial to determine the Fe content of AZ91D than holding time.