<p>With the increasing demand for sustainable resource utilization, the recovery of iron-rich phases from steel slag has garnered significant attention. This study investigated the transformation of iron-bearing phases in synthetic steel slag through thermodynamic analysis. The crystallization behavior of magnesium ferrite spinel (MgFe₂O₄) was explored using in situ high-temperature laser scanning confocal microscopy (HLSCM), phase analysis, and microstructural characterization. The results indicate that an appropriate basicity promotes the transformation of FeO into strongly magnetic MgFe₂O₄, with an optimal basicity of 2.00. Under oxidation conditions, during the cooling process of molten synthetic slag, MgFe₂O₄ crystal nuclei preferentially precipitate at higher temperatures. Subsequently, epitaxial and vertical growth occurs, eventually forming polyhedral structures. As the temperature further decreases to 1350℃, grain migration and aggregation dominate, leading to further growth of MgFe₂O₄ grains. SEM-EDS and XRD analysis further confirmed the transformation of iron-bearing phases into MgFe₂O₄ and its crystallization behavior. This study reveals the key factors influencing the transformation of iron-bearing phases in molten steel slag and the growth control mechanisms of MgFe₂O₄ grains. It provides a theoretical basis for optimizing steel slag oxidation processes to improve iron recovery rates, offering new insights for the resource utilization of industrial solid waste.</p>