<p>Lithium slag (LS) is a solid waste by-product generated during the extraction of lithium salts from spodumene, with its annual output steadily increasing. Recycling LS in road engineering presents an opportunity to mitigate environmental pollution and conserve natural resources. This study aims to evaluate the feasibility of partially or fully replacing limestone filler (LF) with LS, in combination with titanate coupling agent-modified basalt fiber (TBF), to develop a composite-modified asphalt mastic for sustainable pavement applications. The two fillers were characterized by X-ray diffraction (XRD), X-ray fluorescence (XRF), scanning electron microscopy (SEM) and Fourier transform infrared spectroscopy (FTIR). Composite asphalt mastics with varying LS substitution ratios were systematically evaluated in terms of basic physical properties, rheological behavior, low-temperature performance, fatigue life, and thermal stability. The results indicated that the rough, porous structure and active silica-alumina composition of LS provided effective filling and reinforcement. When combined with TBF, the system showed improved dispersion and structural stability. The combined presence of LS and TBF enhanced the high-temperature rheological properties and fatigue life of the asphalt mastic. Although low-temperature flexibility decreased, the material still retained a certain deformation capacity. An environmental benefit assessment showed that at 100% substitution, one kilometer of typical asphalt pavement can consume approximately 589.14 tons of LS, save an equivalent amount of LF, and reduce solid waste land occupation by 22.3 m². Replacing LF with LS is both technically feasible and environmentally beneficial, indicating potential for the resource utilization of LS in pavement materials.</p>