Abstract: In this study, a typical granular column collapse scenario is investigated using a numerical approach that couples computational fluid dynamics (CFD) with the discrete element method (DEM). Granular columns with varying fractal dimensions—used to characterize PSD—are simulated in different fluids to examine the influence of PSD on flow behavior and energy evolution across distinct flow regimes. The results show that as the ambient fluid gradually changes from air to low-viscosity fluid and then to high-viscosity fluid, the runout distance of the granular systems decreases by about 40% compared to dry conditions, and the mobility is significantly weakened. In free-fall and inertial regimes, the mobility difference between systems with different fractal dimensions is only about 1%, whereas in viscous regimes, the system with a higher fractal dimension shows a significant movement delay and the mobility is reduced by 11%. This reduced mobility can be attributed to the increased presence of fine particles, which enhance energy dissipation under low Stokes number conditions. Permeability tests further reveal that mobility is primarily governed by the initial permeability of the immersed granular system—lower permeability corresponds to reduced mobility.