Slopes of open-pit mine dumps face the dual challenges of rapid rainfall infiltration and low water retention capacity during intense rainfall, making them highly susceptible to engineering geological hazards, including soil erosion and landslides. This study investigates the hydro-mechanical regulation mechanisms of fly ash (FA) and polyacrylamide (PAM) amendments through multi-scale experiments, including simulated rainfall experiments, moisture monitoring, triaxial shear tests, soil-water characteristic curve (SWCC) analyses, and microstructural observations. The results indicate that FA improves pore structure through hydration-induced cementation and particle filling, while PAM forms a polymer network through adsorption, bonding, and flocculation, thereby delaying rainfall infiltration. The FA-PAM composite system significantly retards wetting front migration through multiple synergistic mechanisms, including particle filling, cementation, adsorption, bonding, viscosity enhancement, bridging, and polymer flocculation. The soil structure transforms from a loose-fissured state to a dense-cemented state, strengthening skeletal continuity and structural integrity, enhancing soil strength, and increasing both the air-entry value and residual saturation of the SWCC, thereby achieving synchronized optimization of hydraulic regulation and structural reinforcement. A method for identifying SWCC parameters was proposed. The critical hydraulic state boundary was determined, thereby elucidating the migration mechanisms of pore water across the boundary, transition, and residual zones. Overall, these findings advance the theoretical understanding of the hydro-mechanical evolution of improved unsaturated soils and offer a transferable engineering strategy for erosion control and landslide mitigation on coarse-grained slopes in open-pit mine dumps and other arid and semi-arid regions.