The knowledge on distribution of mineralogical phase in slag is of vital importance to disposal and utilization of slag. Despite of certain amount (up to 5%) of MnO found in stainless steel slag, MnO distribution in phases of slag was not thoroughly investigated. In this work, phases of synthetic CaO–Al2O3–MgO–SiO2–CrOx–MnO slag with varied CaO/SiO2 and MnO content, which was soaked at 1673 K for 12 h, were determined by combining SEM-EDS and XRD technique. Phases for samples with CaO/SiO2 ratio of 1.5 and 1.3 were composed of Ca2SiO4, merwinite, spinel, and liquid phase, while phases for samples with CaO/SiO2 ratio of 1.1 are composed of spinel and liquid phase. The calculated phases by FactSage were in good agreements with observed ones by SEM-EDS and XRD. Manganese was found to be enriched in spinel and liquid, and chromium is enriched in spinel, and only negligible Cr could be found in merwinite and Ca2SiO4. The spinel increases in size with increasing of MnO and decreasing of CaO/SiO2 ratio. Disintegration was found in samples with increased MnO content and CaO/SiO2 ratio. The size of Ca2SiO4 crystal increases in size with increase of MnO content and CaO/SiO2 ratio, which could interpret the disintegration of slag. The present investigation indicates that MnO content has important effects on disintegration and spinel precipitation in stainless steel slag.