<p>Basicity is one of the key factors affecting the property of slag from the double slag converter steelmaking process. The influence of basicity on the viscosity and crystallization of slag from the double slag converter steelmaking process were evaluated in this study, and it was found that the characteristic temperature of slag tended to increase and then decrease with the increase of basicity. As basicity increased, the viscosity of slag initially increased, followed by a decrease, reaching its maximum value when the basicity was 1.7. The FactSage thermodynamic software was used to calculate the composition and content of precipitated phase of slag in the crystallization process. The phase composition and micromorphology of slag were investigated by X-ray diffraction (XRD) and scanning electron microscope with energy dispersive spectroscopy (SEM-EDS). The results showed that, when the basicity was below 1.5, the precipitated contents of Ca₅P₂SiO₁₂ and Ca₂SiO₄ increased with the increase of basicity. As the basicity exceeded 1.5, the precipitated content of Ca₂SiO₄ increased with increasing basicity, while Ca₅P₂SiO₁₂ was not observed to increase. The content of P-rich phase increased and the content of matrix phase decreased with increasing basicity. These findings could contribute to a deeper understanding of the viscosity and crystallization of slag from the double slag converter steelmaking process.</p>