High brittleness, inadequate compressive capacity, severe muddying under rainfall, and insufficient shear resistance significantly compromise the serviceability of base layers in heavy-haul mining roads. To address these issues, this study proposes a green and low-carbon stabilization strategy in which a nano-silica-based material and polypropylene (PP) fibers are synergistically incorporated into a soil base. An L9 (34) orthogonal experimental program was conducted to quantify the effects of nano-silica dosage, fiber content, and fiber length on unconfined compressive strength (UCS), direct shear strength (DSS), and the water-stability coefficient. Furthermore, a three-dimensional road structural model was developed in ABAQUS to analyze the mechanical responses of different base-material alternatives coupled with varying surface-layer thicknesses under mining-truck wheel loading. The results show that the nano-silica-based material markedly improves UCS and water stability mainly through micro-pore filling and chemically promoted cementation; at a dosage of 3%, the UCS reached a peak value of 4.04 MPa. In contrast, PP fibers primarily enhance shear performance via a three-dimensional bridging mechanism, achieving a maximum DSS of 145.7 kPa. Multi-objective optimization identified an optimal mix proportion of 3% nano-silica-based material, 0.5% fiber content, and a fiber length of 12 mm, leading to a synergistic improvement in strength and toughness. Numerical simulations indicate that the higher elastic modulus of the composite-improved soil effectively reduces the peak settlement within wheel tracks. The vertical stress at the subgrade surface is governed mainly by the surface-layer thickness; increasing the thickness from 20 cm to 40 cm and 60 cm reduces the transmitted peak stress by approximately 30% and 50%, respectively. Accordingly, the composite-improved soil is recommended for engineering practice, together with a structural layer thickness exceeding 40 cm, to achieve dual control of pavement deformation and subgrade stress.