<p>Unsteady flows of granular materials immersed in viscous fluids are commonly encountered in natural and industrial processes, exhibiting pronounced sensitivity to intrinsic properties of both particles and fluids, such as the particle–fluid density ratio. Understanding their complex dynamics and the underlying mechanisms, especially the intricate particle–fluid interactions at the pore scale, is challenging but crucial. In this study, a high-performance framework coupling the lattice Boltzmann method and discrete element method was employed for immersed granular flows. Experimental measurements of buoyant granular column collapses were taken to validate the accuracy and efficiency of the coupling algorithm. The influence of particle–fluid density ratios on the collapse dynamics of immersed granular columns was explored through macro- and microscopic analysis. Results show that collapses exhibit larger front positions and more efficient energy conversions as particle–fluid density contrasts increase, with floating particles achieving longer runout distances and higher front velocities compared to settling cases. Micromechanical analysis reveals that in settling scenarios, an enhanced contact force network can be formed to inhibit particle sliding during the initial stage, leading to longer collapse durations and slower dynamics, whereas floating columns tend to experience catastrophic failures. Furthermore, the particle Stokes number (<i>St</i>) serves as a governing parameter for immersed granular flows, exhibiting a strong power-law correlation with the normalized runout distance across various length scales.</p>