Behaviors and evolutions of MgO•Al₂O₃ in non-oriented silicon steel during calcium treatment were investigated by thermodynamic calculations and high-temperature experiments. MgO·A l₂O₃ inclusions were the main type of inclusions before calcium treatment, which is also confirmed by the stability diagram of MgO/MgO· Al₂O₃/Al₂O₃ from thermodynamic calculations. After calcium treatment, the majority of original MgO·Al2O3 inclusions were mainly modified to CaO-MgO-Al₂O₃ system inclusions, this would be beneficial to improve castability and product quality of silicon steel. But there was no modification happening to some MgO·Al₂O₃ spinels which were directly surrounded by CaS inclusions. The content of Al₂O₃ occasionally existed a sudden increase besides a common decrease in the product layer of CaOMgO· Al₂O₃. Then the evolution mechanisms of MgO·Al₂O₃ inclusions were proposed in present work. Thus, Al/Mg atom ratio had a significant influence on the modification of MgO·Al₂O₃ spinel.