Magnesium oxide solid dissolved in silicate phase, mesophase and RO-phase which are the major constituents of steel slag. The MgO exists in the steel slag with complex forms, which results in the bad soundness of the steel slag. In this paper, the silicate phase and mesophase were separated by salicylic acid methanol solution and potassium hydroxide-sucrose solution respectively and were studied combining with the XRD and SEM. The results show that the silicate phase and mesophase can be fully dissolved, however, the RO phase is not influenced by the two solutions in this process. The content of magnesium oxide existed in different minerals can be detected by chemical analysis. It can be found that the magnesium oxide mainly exists as RO-phase in steel slag, and some of them exist as free state, while only a small quantity of solid dissolve in the silicate phase and mesophase.