To determine the effect of the CaO/Al 2 O 3 ratio on desulphurization and inclusion evolution in low-density steel, a series of laboratory-scale experiments are performed. The results show that violent reaction occurs at the interface between refining slag and molten steel. As the slag-steel reaction progresses, (CaO) and (MgO) are reduced to Ca and Mg by dissolved Al. The deoxidation product Al 2 O 3 dissolves into the slag and accumulates at the slag-steel interface. The evolution process of the sulphide inclusions is CaS → MgS → MnS. The sulphide outer layer wraps AlN and spinel inclusions, floats up and is absorbed by the top slag. During refining process, the accumulated Al 2 O 3 at the slag-steel interface easily forms MgO·Al 2 O 3 . Finally, a dense spinel layer can be formed at the slag-steel interface, which inhibits the desulphurization reaction. Reducing the C/A ratio can reduce the formation of MgO·Al 2 O 3 and has a positive effect on the desulphurization in low-density steel. The influence of the C/A ratio on desulphurization kinetics should be considered in the design of slag composition.