<p>Steelmaking slag, as the main solid waste in the metallurgical industry, not only produces phosphorus fertilizer but also is reused as metallurgical flux. In this study, the effect of Al₂O₃ on the crystallization process and subsequent phosphorus leaching behavior of steelmaking slag was investigated. The phase transformation of steelmaking slag during the cooling process was studied by FactSage thermodynamic software in combination with X-ray diffraction and scanning electron microscopy. The results showed that with the increase of Al₂O₃ content, the precipitation temperature of Ca₅P₂SiO₁₂ (C₂S–C₃P) solid solution decreased gradually, and the amount of precipitation showed a trend of increasing and then decreasing, with the maximum value between 6.39 and 8.39%. The element distribution became more concentrated and the enriched phase area of phosphorus increased significantly. Under the conditions of leaching temperature of 25&#xa0;°C, leaching time of 90&#xa0;min, leaching acid of citric acid, pH of 6, liquid–solid ratio of 400:1, and stirring rate of 200&#xa0;rpm, the leaching efficiency of phosphorus showed a tendency of increasing and then decreasing with the increase of Al₂O₃ content. Once the Al₂O₃ content exceeded 6.39%, it will significantly inhibit the formation of C₂S–C₃P solid solution, thereby affecting the subsequent leaching of phosphorus. This study provides insights for optimizing the leaching process and offers a theoretical and technical foundation for the utilization of steelmaking slag.</p> Graphical Abstract <p></p>