Abstract: The flocculation and sedimentation of coal tailing water is one of the key factors affecting the efficient and stable of coal preparation plants, while the fluid flow of the flocculation device exerts a significant influence on the flocculation process and its effectiveness. To develop a high-efficiency coal tailing water flocculation device, a down-flow grid flocculator with cylindrical grid bar was designed with reference to grid flocculators in the water treatment field. Computational Fluid Dynamics (CFD) and the CFD-Discrete Element Method (CFD-DEM) were employed to simulate the distribution of hydrodynamic parameters in the flow field of the grid and the collision-adhesion behaviors of particles in the flow field, thereby identifying the most critical parameters affecting particle collision. Taking these key parameters as evaluation indices, the CFD method was further used to optimize the structure of the grid flocculation device. Finally, CFD-DEM simulations and experiments of coal slime water flocculation were carried out on the optimized grid device to verify the rationality of the research. The results show that in the grid flocculation device, the grid bar induces turbulent fluctuations of the fluid and generates trailing vortices, which increase the turbulent kinetic energy (TKE) and turbulent kinetic energy dissipation rate (TKEDR) of the fluid, thereby enhancing the collision probability of coal slime water particles. Among the influencing factors, TKE and TKEDR are the two key parameters most closely related to particle collision probability. However, the TKEDR caused by friction between the fluid and the grid surface is ineffective dissipation and exerts no significant promotion effect on particle collision. The grid diameter (d), intra-row distance-diameter ratio (l/d), and inter-row distance-diameter ratio (h/d) have important impacts on TKE and TKEDR. Within the research range, the optimal grid diameter is 4 mm. When l/d ranges from 2.0 to 2.3, both TKE and TKEDR increase with the rise of l/d and reach their maximum values at l/d = 2.3; with a further increase in l/d, both parameters decrease accordingly. As the inter-row distance-diameter ratio h/d increases from small to large values, TKE and TKEDR also increase first and then decrease, achieving the optimal values at h/d = 3.5. In the optimized grid flocculation device, the variation in the number of particles contained in the flocs obtained from CFD-DEM simulations shows good consistency with the variation in floc particle size measured in experiments, indicating the rationality of the simulation results. The flocculation growth rate in the grid region of the flocculation device is nearly linear, which is significantly better than that in the non-grid region, demonstrating that the optimized grid structure has a favorable promotion effect on particle flocculation.