Mixed fluid-particle flows are commonly found in nature and exhibit complex particle-particle and particle-fluid interactions. In this paper, a typical small-scale case of immersed granular collapse under the viscous regime is numerically investigated using computational fluid dynamics coupled with the discrete element method (CFD-DEM), which provide particle-scale information of the collapse. The input parameters for the coupled CFD-DEM model are carefully calibrated from experimental results, and the simulation results achieve good agreement with the experiments in terms of the front evolution and final deposition. The collapse processes for different aspect ratios exhibit similarities and propagate in a three-stage mode that includes acceleration, steady propagation, and deceleration. The propagation velocity, runout distance, and the energy evolution of both fluid and particles are presented. The final runout is linearly proportional to the densimetric Froude number in our high-column cases. The transition of particles' motion from vertical to horizontal and the drag of the fluid are found to be responsible for the constant velocity in the steady propagation stage. We also show that a small energy bump during the initial stage is the result of particle destabilization and rearrangement. Published under license by AIP Publishing.