Abstract: In order to improve the high temperature properties of low grade magnesium-based material, Mg-α-SiAlON/MgO composites were prepared using sintered magnesia, Al metal powder, Si powder and Al₂O₃ powder as the raw material, the Mg-α-SiAlON phase in-situ forming in the Mg-α-SiAlON/MgO composites sintered at different temperature and mass fraction were determined by XRD, SEM and EDS. The synthesis atmosphere conditions of Mg-α-SiAlON were determined by thermodynamic analysis and the reaction mechanism of Mg-α-SiAlON phase were explained. The experiment results show that the partial pressure of nitrogen and oxygen at 1 550℃ satisfies the thermodynamic requirement for the synthesis of Mg-α-SiAlON. When the total mass fraction of sintered magnesia powder, Al metal powder, Si powder and Al₂O₃ powder is 40wt%, the content of Mg-α-SiAlON phase with plate-like shape is as high as 32.1% in Mg-α-SiAlON/MgO composites nitrided at 1 550℃ for 3 h. Mg-α-SiAlON phase interlace to form a skeleton and envelope the low melting phase inside of it. The in-situ synthesis of Mg-α-SiAlON/MgO composites from sintered magnesia by nitridation is beneficial for improving the thermal shock resistance of magnesium-based material.