Abstract: Incoloy825 alloy is extensively used in the aerospace and petrochemical industries owing to its excellent corrosion resistance and mechanical properties. It is a solid solution-strengthened Fe−Cr−Ni-based corrosion-resistant alloy. The changes in the Al and Ti contents of the alloy determine the precipitation temperature of the strengthening phases γ '(Ni3AlTi) and Ti (C, N) in the alloy. At present, the main production methods of Incoloy825 alloy are vacuum melting and electroslag remelting. However, owing to the reaction of the components in the slag with the Al and Ti elements in the alloy during the electroslag remelting process, the axial component distribution of the Al and Ti elements in the electroslag ingot is not homogeneous, which seriously affects the quality of the electroslag ingot. It is necessary to control the Al and Ti contents in Incoloy825 alloy and reduce the volatilization of fluoride during the electroslag remelting process. The thermodynamic model of slag metal reaction was established using FactSage thermodynamic software. A low-fluorine slag system suitable for controlling Al and Ti contents was designed, and the relationship between the components in the slag and the activity ratios of Al₂O₃ and TiO₂ was studied, the result was verified by a high-temperature slag metal equilibrium experiment. The results show that the CaO and Al₂O₃ contents in slag increases with increase in the \lg \left( a_\rmA\rml_\rm2\rmO_\rm3^2 / a_\rmTi\rmO_\rm2^3 \right) value, while the Ti content in the alloy decreases with increasing Al content. Moreover, as the TiO₂ content in the slag increases, the \lg \left( a_\rmA\rml_\rm2\rmO_\rm3^2 / a_\rmTi\rmO_\rm2^3 \right) value decreases, Ti content increases and Al content decreases. The CaF₂ and MgO contents in the slag increase have a little effect with the \lg \left( a_\rmA\rml_\rm2\rmO_\rm3^2 / a_\rmTi\rmO_\rm2^3 \right)value. When the difference between the Al and Ti contents in the alloy is large, the elemental Ti in the alloy is easy to be oxidized; when difference between the Al and Ti contents is small, the elemental Al is easy to be oxidized. When the mass percent of CaO and Al₂O₃ in the slag are 30%−33% respectively, the mass percent of TiO₂ is 6%−12%, the mass percent of CaF₂ is 20%−30%, the mass percent of MgO is 1%−5%, the Al and Ti contents in the alloy can be controlled.