Abstract: Aiming at the defects of loose structure and extremely poor water retention capacity of graphite tailings (GT) in engineering and ecological restoration, a composite modification approach integrating biomass fiber (BF), polyacrylamide (PAM), and diatomite (DE) is proposed in this study. The optimal proportion of the composite modifier was determined through orthogonal experiments. The soil-water characteristics of the graphite tailings matrix soil (GTMS) under different dry densities and particle sizes were evaluated using the contact filter paper method. Furthermore, the microscopic pore reconstruction mechanism was revealed by combining scanning electron microscopy (SEM) and PCAS technology, and the hydraulic evolution characteristics were fitted and analyzed using the Van Genuchten (VG) model. The results indicate that the water retention performance of the GTMS reaches a local optimum at a modifier dosage of 4% BF, 0.16% PAM, and 1.5% DE. As the dry density increases from 1.40 g/cm³ to 1.70 g/cm³, the air-entry value rises significantly from 2.77 kPa to 10.20 kPa. With the decrease in particle size, the saturated volumetric water content gradually increases from 48.48% to 56.06%, exhibiting a remarkable enhancement in water retention capacity. Microscopic analysis reveals that through gel encapsulation and the fiber network, the composite modifier exerts a multiphase synergistic reconstruction effect—specifically, physical fiber support, interfacial gel cross-linking, and mineral micropore filling—which drives the transformation of GT from a single-grain structure to a dense aggregate structure. The VG model verifies the macroscopic barrier mechanism driven by microscopic pore optimization: pore densification leads to a substantial increase in the air-entry value characterizing the maximum pore feature, thereby significantly elevating the energy threshold required for water to break through the pores. Moreover, the evolution laws of the model parameters (n and α) are highly consistent with the microscopic surface porosity. The research findings demonstrate that the composite modification can significantly improve the water retention performance of GT, providing a theoretical basis and practical reference for the application of the BF-PAM-DE composite system in GTMS reconstruction and ecological restoration of mining areas.