Abstract: The Ruhrstahl Heraeus (RH) refining furnace is a piece of important secondary refining equipment that is widely used in the production of special steel owing to its high efficiency of degassing, decarburization, and de-intercalation. However, molten steel that has a high alloy content will encounter key problems in the vacuum treatment process, and the loss of volatile alloying elements in the molten steel is considerable, resulting in the nodulation of the molten steel vacuum splashing and secondary oxidation of the subsequent molten steel. To address the problems of elemental loss and vacuum splashing caused by manganese (Mn) gasification during the vacuum processing of manganese-containing steel using RH, the variation and migration behavior of Mn in molten steel under different vacuum treatment conditions of 120 t RH were examined. This study analyzed the relationship between manganese elemental loss and its volatilization and vacuum splattering, and it was verified in an anatomical experiment of the nodule at different positions inside the RH vacuum chamber. The results show that elemental Mn in the molten steel shows obvious loss during the vacuum process of RH, and the loss in the early stage of the vacuum process is the largest. The composition of manganese oxide in the nodule of the RH vacuum chamber is as high as 14%–70%, and the thermodynamic calculation results show that temperature, the content of Mn in the steel, and the degree of vacuum have a considerable influence on the volatilization behavior of Mn, which is the key influencing factor for manganese migration during the vacuum process. By improving the vacuum pressure drop mode, a stepwise vacuum is used to reduce the loss of elemental Mn from the original 2×10⁻⁴ to 1×10⁻⁴. The results have considerable significance for on-site production, and steel can be effectively restrained by improving the vacuum pressure drop mode. Additionally, the splashing and volatilization of liquid reduces the loss of the alloying element Mn.