Abstract: Thermodynamics on the formation and transformation of spinel inclusions in gas cylinder steel and the effect of calcium treatment on the compositions of molten steel and inclusions were studied by using Factsage 6.0 software. The results show that spinel inclusions will be produced even through the content of magnesium is very low, and the spinel inclusions will transform to liquid complex inclusions when the content of dissolved Ca in the molten steel exceeds 1×10^-6. With the increase of calcium, on one hand, the contents of Al₂O₃ and MgO in the complex inclusion keep decreasing, and the content of CaO increases, but the content of SiO₂ changes a little. On the other hand, the content of oxygen decreases, while the content magnesium increases.