The better understanding of structural effect of composition is of primary importance in the design of converter slag and for rationalizing the foaming performance of smelting process. In the present work, the CaO–SiO₂–Fe_xO samples with different compositions were prepared to simulate the converter slag of initial smelting stage. The compositions and structural units of slag samples were investigated by combining X-ray fluorescence spectroscopy and Raman spectroscopy. According to the results, the transformation behaviors of structural units and the degree of polymerization (DOP) of molten slag were further analyzed. The results of Raman spectra showed that when basicity increased from 0.38 to 0.97 and total iron content decreased from 32.77 to 13.26 mass%, increasing O²⁻ led to the depolymerization of [SiO₄]⁴⁻ tetrahedrons from Q³ to Q⁰ units and the increasing [FeO₄]⁵⁻/[FeO₆]⁹⁻ ratio. With further increasing basicity from 0.97 to 1.25, Q³ units disappeared and more O²⁻ reacted with [FeO₄]⁵⁻ tetrahedrons to form [FeO₆]⁹⁻ octahedrons. Meanwhile, Fe³⁺ could probably form Si–O–Fe bond by replacing Si⁴⁺ cations in Q³ units. Overall, the depolymerization of [SiO₄]⁴⁻ tetrahedrons from Q³ to Q⁰ units was the main reason for the decreasing DOP of molten slag during the early stage of basic oxygen steelmaking.