The composition characteristics and formation mechanism are analyzed for magnesium-bearing inclusions in the steel by sampling and thermodynamic calculation. The results show the main oxides is MgS precipitating with 2MgO·SiO₂ or MgO-Al₂O₃-SiO₂. And MgS and AlN are the main precipitates. MgS-bearing compound oxide inclusions are mainly globular within 1 μm to 2 μm in size. MgS restricts MnS precipitation, and any type of MnS isn't found in the steel. MgS-bearing inclusions exist in the steel with three forms:MgS wrapped with spherical oxide core, MgS uniformly distributed on the surface of spherical oxides, and MgS orientedly precipitated on oxides matrix or with AlN. Thermodynamic calculation results indicate magnesium content and characteristics of oxides decide the composition of inclusions in the steel. MgS is easy to precipitate with MgO-Al₂O₃-SiO₂ complex with high MgO-bearing and sulphur capacity. The size of inclusions forming by MgS orienteering precipitating on oxides matrix or with AlN is much bigger, and their precipitation and growth method is related with the crystal structure of MgS and inclusions matrix. Fine MnS and MgO·Al₂O₃ can be diminished by controlling MgO content in refining slag to form composite inclusions bearing MgS.