Abstract: The process of preparing metallic magnesium and magnesium aluminum spinel with magnesite as the raw material by the aluminothermic method under relative vacuum was studied. The effects of carrier gas flow, briquetting pressure, calcination temperature, aluminum content, reduction temperature and the time on the reduction rate of magnesium oxide, as well as the effects of secondary high-temperature treatment on magnesium aluminum spinel were systematically studied. The phase composition and microstructure of the reduction slag were analyzed by X-ray diffraction (XRD) and scanning electron microscopy with energy dispersive spectroscopy (SEM-EDS). The results showed that a relative vacuum was suitable for the aluminothermic reduction of the magnesium oxide system. When the carrier gas flow was 0.2 m³/h, the pelleting pressure was 15 MPa, and ω（Al）∶ω（2MgO）=1.2, the reduction rate of magnesium oxide exceeded 70% at 1 523 K for 1 h. After secondary high-temperature treatment at 1 923 K, the reduction product increased the grain size of spinel from 0.4 μm to 10.0 μm, increased the content from 78% to 93%, and significantly improved the product’s crystallinity.