Abstract: To achieve the comprehensive utilization of coal slime resources and meet the stringent requirements for moisture content during recovery and reuse, the core challenge of efficient coal slime dewatering must first be addressed. This study proposes the use of polydiallyldimethylammonium chloride (PDDA) as a novel filter aid, working synergistically with ultra-high pressure filtration (UHPF) technology. Through SEM-EDS characterization and fractal theory analysis, combined with multi-scale 3D CT scanning, pore network modeling (PNM), low-field nuclear magnetic resonance (LF-NMR) testing, and COMSOL seepage simulation, the pivotal role of PDDA in optimizing pore-throat structures, improving pore connectivity, and reducing seepage resistance was elucidated. Laboratory and semi-industrial tests demonstrate that at a PDDA-15040 dosage of 350 g/t, the coal slime floc size increases to approximately 83 μm, and the moisture content of the coal slime is reduced to 19.24% under a pressure of 10 MPa. Multi-scale 3D CT reconstruction and PNM analysis reveal that PDDA expands the pore space while reducing the pore-throat ratio and tortuosity. Specifically, it increases the average coordination number of the filter cake pores from 2.68 to 3.86, thereby enhancing pore connectivity. LF-NMR analysis, combined with Fourier-transform infrared spectroscopy (FTIR), reveals the occurrence states and migration laws of water within the filter cake. PDDA weakens the binding force between coal particles and water molecules, reducing the content of bound and adsorbed water. COMSOL seepage simulations further indicate that PDDA significantly improves the permeability of the filter cake, optimizes the pressure gradient distribution, and constructs a continuous network of seepage channels. This study achieves precise regulation of the filter cake microstructure induced by PDDA, ultimately proposing a cost-effective coal slime dewatering strategy. This provides reliable technical support for the recovery and utilization of coal slime as a low-calorific-value fuel, building material, and fertilizer feedstock.