There are four types of Mg–Al–Si–O inclusions observed in the Mg-treated Al-deoxidized ultra-low-carbon steel con-taining trace Si, including SiO2, 2MgO·SiO2, 3Al2O3·2SiO2, and 2MgO·2Al2O3·5SiO2 and their composite inclusions. Using FactSage, the phase relationship of Mg–Al–Si–O system at 1473–2073 K was calculated using FactSage software, and the change in Si content can change the stable region range of different Mg–Al–Si–O-based inclusions. Based on the types of inclusions observed experimentally, the formation pathways of inclusions were predicted and a kinetic model was established to describe the formation process of the xMgO·yAl2O3–Mg–Al–Si–O and xAl2O3·ySiO2–Mg–Al–Si–O inclusions.