<p>Nonmetallic inclusions in 316 stainless steel for nuclear power plant adversely affect its corrosion resistance, fatigue properties, and strength. In this work, the 316 stainless steel for nuclear power plant produced by silicon and aluminum composite deoxidation process was used as the research object, and the evolution mechanism of nonmetallic inclusions in steel was investigated to guide the control of inclusions. Through the whole process sampling, the evolution of inclusion types in the silicon aluminum composite deoxidation process was described as MnO-Cr₂O₃ → SiO₂-MnO-Cr₂O₃ → Al₂O₃-SiO₂-MnO → Al₂O₃-SiO₂-CaO → Al₂O₃-SiO₂-CaO-CaS. Then, the thermodynamic software FactSage was utilized to calculate the impact of oxygen, aluminum, and calcium contents, as well as temperature, on the transition of inclusion types. The findings indicated that SiO₂-MnO-Cr₂O₃ inclusions were generated in steel when the oxygen content exceeded 0.065%. Moreover, regulating the calcium content in steel to below 0.0055% could effectively impede the formation of CaS inclusions. The temperature markedly influenced the formation of inclusions during steel solidification, as the temperature diminished, the content of Al₂O₃ and CaS in the inclusions increased. In contrast, the levels of SiO₂ and CaO decreased.</p> Graphical Abstract <p></p>