Abstract: The performance of Fe-Cr-Al alloy can be improved significantly when it contains a small amount of rare earth. In this paper, after studying the slags used for producing Fe-Cr-Al alloy with the method of ESR, we then establish the thermodynamic model of mass action concentration of CaF₂-Al₂O₃-CaO-Ce₂O₃ at 1873 K based on ion and molecule coexistence theory (IMCT). The result demonstrates that the value of Ce₂O₃·Al₂O₃ activity increases when keeping and increasing Ce₂O₃ content in slags, but the phase composition in slags does not change; when the concentration of Ce₂O₃ is 10 %, 20 %, 30 %, 40 % respectively, the value of differs and poses different impacts on the activity of compositions, but when the value is between 1.0 and 1.8, the activity of CaO increases fastest and that of Ce₂O₃·Al₂O₃ decreases, which demonstrates the decomposition role of CaO in slags is remarkable. We select slags to conduct melting and XRD experiment on the samples and then use jade5.0 software to analyze the result, which proves to be consistent with the theory.