The leaching toxicity of lead in rare earth smelting slag is significantly high. Taking rare earth smelting slags (S1, S2, S3) from three Sichuan-based rare earth enterprises as research objects, this study conducted laboratory leaching experiments to investigate the immobilization effects of phosphorus-based materials and alkaline substances, both individually and in combination on lead in rare earth smelting slag, while screening and optimizing the dosage and ratio of phosphorus-based materials to alkaline substances. The results demonstrate that the sole addition of phosphorus-based materials can significantly reduce the leaching concentration of lead. For instance, adding 5% calcium superphosphate (by slag mass) can decrease the lead leaching concentration in S1 from 485 mg/L to 70.2 mg/L. However, this approach will cause pH reduction in slag, potentially inducing secondary pollution. While sole application of alkaline substances also notably reduce lead leaching concentration, excessive dosing risked excessively high pH levels. The combined use of phosphorus-based materials and alkaline substances can synergistically regulate slag pH to 9~10 while effectively reducing lead leaching concentration, achieving significantly superior outcomes compared to single treatments. The phosphorus-alkaline combined approach not only mitigate lead migration risks but also minimize environmental hazards caused by chemical overdosing. These findings provide theoretical basis and technical references for the remediation of multi-metal composite pollution in rare earth smelting slags.