Abstract: In order to develop new type magnesia refractories, aluminum powder, silicon powder and fused magnesia powder were used as the raw matters, the polytypic Mg-sialon phase forming in the composite material of MgO, Al and Si sintered at 1350℃ and 1550℃ in oxidizing, reducing and nitrogen atmosphere were characterized by XRD, SEM and energy dispersion spectrum (EDS) respectively, and the formation mechanism of polytypic Mg-sialon phase were explained by thermodynamic analysis at the same time. The experiment result shows that after sintering in both oxidizing, reducing or nitrogen atmosphere, Al and Si in sintered composites mainly translate into Al₂O₃, AlN and SiO₂ by gas-solid/liquid reaction, and then react with MgO to form polytypic Mg-sialon phase. The polytypic Mg-sialon phase in-situ synthesized in Mg-sialon/MgO composite of MgO, Al and Si sintered in three different atmospheres all show plate-like shape, and it is mainly forms in the closed pores of the specimen sintered in oxidizing atmosphere, holes left behind by the molten metal in the specimen sintered in reducing atmosphere and whole area of the specimen sintered in nitrogen atmosphere.