Abstract: To clarify the evolution behavior of nonmetallic inclusions in gear steel in the whole process, achieve effective control of the characteristics of inclusions in steel and improve product quality, 20CrMnTiH gear steel was taken as the research object. It was sampled in the "BOF-CAS-LF-VD-CC-rolling" smelting process, and SEM was used to systematically analyze and study the composition, size, morphology and quantity of its inclusions at different stages. The results showed that after the converter smelting and before the CAS refining, there were only Al₂O₃ inclusions in clusters in the steel. When the LF white slag was produced for 5 min, calcium wire was added for Ca treatment, which promoted the morphological change of the inclusions, forming Al₂O₃, Al₂O₃-CaO inclusions and Al₂O₃-MgO inclusions. When the LF exited the station, it had been combined into spherical Al₂O₃-MgO-CaO and CaS-MnS composite inclusions and TiN inclusions. When in the middle package position, in addition to spherical Al₂O₃-MgO-CaO composite inclusions and CaS inclusions, there were also fine and small-sized oxidized inclusions.