The in-situ formation mechanism of MgAl2O4 was introduced, focusing on the formation process by solid phase reaction and gas phase reaction as well as the phenomenon of secondary spinelization. The influencing factors of the in-situ MgAl2O4 formation and its effect on the microstructure and the properties of materials were systematically summarized for the Al2O3- MgO-MgAl2O4 system and the carbon-containing refractories systems. It was pointed out that the in-situ formation of MgAl2O4, including secondary spinelization, can regulate the microstructure and the service performance of materials. Its expansion effect can not only offset the shrinkage caused by sintering to improve the corrosion resistance of refractories, but also seriously restrict the reliability of functional refractories. The composition, the particle size, the atmosphere, and the temperature are important factors affecting the in-situ formation of MgAl2O4. In the carbon-containing materials systems, the solid-solid reaction and the gas-solid reaction coexist to produce MgAl2O4, which provides an effective way to further regulate the microstructure and the properties of materials through the reaction process.