The accumulation or landfill of lithium slag will contaminate the surrounding soil and water quality with residual sulfides and harmful elements, causing serious environmental hazards. This study aims to use Lithium slag (LS) as a sustainable alternative for silica flour (SF) in high-temperature cementing and examines the effects of this substitution on the microstructural and mechanical properties of cement pastes. The results show that an appropriate amount of LS can reduce the permeability of oil well cement and increase its high temperature compressive strength. Compared with pure paste (RS), the compressive strength of the sample replaced by 30 % LS increased by 87.8 % and the permeability decreased by 57.1 % after 28 days of high temperature curing. From the phase point of view, the samples supplemented with LS can form Xonotlite and Katoite with dense structure and high temperature stability. These hydration products can reduce the matrix porosity and permeability, increase the matrix density, and effectively improve the compressive strength of the cement pastes. In addition, the environmental effect analysis showed that the leaching toxicity and radioactivity of the sample did not exceed the standard requirements. This study provides a new direction for the sustainable utilization of LS resources, which not only combats the environmental pollution caused by LS accumulation, but also reduces the cost of cementing materials.