<p>Recently, with the large-scale generation of lithium slag (LS), its safe disposal has emerged as a critical focus across multiple disciplines. This study investigated the potential application of lepidolite LS as a high temperature stabilizer in cementing at high temperature and high pressure (HTHP). This paper investigated the effect of LS on the compressive strength and microstructure of hardened cement pastes (HCPs) at 260℃ and 21 MPa using X-ray diffraction (XRD), scanning electron microscopy with energy-dispersive spectroscopy (SEM–EDS), ²⁹Si and ²⁷Al magic-angle spinning nuclear magnetic resonance (MAS-NMR). Results showed that the incorporation of LS significantly altered the phase composition of HCPs, promoting the formation of hibschite. However, LS alone failed to prevent strength retrogression, a phenomenon where the compressive strength of HCPs decreases after prolonged exposure to HTHP. Partial replacement of LS or cement with silica flour (5–10 wt%) optimized performance, yielding HCPs with stable compressive strength (&gt; 14 MPa) and minimal retrogression (0.86%). The phase separation in calcium-aluminosilicate hydrate (C-A-S–H), forming wollastonite and analcime, enhanced strength stability. This separation is energetically favored, driven by the reduction of excess enthalpy in the high-Al system upon silica addition. The leaching concentrations of heavy metal ions from the cement pastes were evaluated following the Chinese standard GB/T 5085.3 (acid digestion method) and found to be far below the regulatory limits.</p>