Abstract: Coal is the primary energy source in China. And coal separation is the initial technology for clean utilization of coal, which can effectively improve coal quality and utilization efficiency. Coal resources and water resources are inversely distributed in China. It difficult to apply the high water consumption wet separation technology in arid regions. It is of significance to study the efficient dry coal separation technology. Gas-solid separation fluidized bed is one of the key technologies for improving coal quality. Nevertheless, conventional Geldart B/D binary dense medium particles suffer from severe bed density fluctuations, a narrow adjustment range, and poor fine-coal separation accuracy during fluidization, which seriously hinder the scale-up and industrial popularization of separation fluidized beds. To address these bottlenecks, this study designed and developed a novel Geldart A⁻ ultrafine composite dense medium particle system, using Geldart A magnetite powder as the base heavy medium and adding Geldart C ultrafine coal fines for composite modification, so as to enhance the fluidization process and achieve wide-range density regulation. The study systematically investigated the modification mechanism of the composite dense medium, microscopic flow characteristics, bubble and particle motion behaviors, bed expansion features, and density distribution patterns. The results show that the ultrafine coal fines effectively reduce interparticle forces through surface adhesion and void filling, increasing the emulsion phase expansion ratio from 0.13 to 0.31 and the overall bed expansion ratio from 0.17 to 0.35; the average bed density can be stably adjusted within the range of 1.5–2.1 g/cm³, significantly broadening the regulation window compared with traditional systems, and improving bed density uniformity and fluidization stability. This provides technical support for promoting the large-scale and practical industrialization of Geldart A/A⁻ fine composite dense medium separation fluidized bed equipment.