This study utilized lithium slag and mineral powder to prepare alkali-activated composite cementitious materials, aimed at addressing the disposal issues of lithium slag and reducing the energy consumption and CO₂ emissions from cement production. The research primarily analyzed the effects of water-binder ratio and lithium slag content on the rheological properties of materials, and further explored the effects of different calcination temperatures and lithium slag content on the mechanical properties of materials. The findings indicate that the fluidity of the alkali-activated lithium slag composite cementitious materials increases with the water-binder ratio, but decreases as the lithium slag content increases. The optimal calcination temperature for lithium slag is 700 ℃, under which the alkali-activated lithium slag composite cementitious materials exhibit the best flexural and compressive strength at 90 d. 25% (mass fraction) lithium slag content is the optimal proportion for the alkali-activated composite cementitious materials, as excessive content inhibits the hydration reaction. Compared to traditional cement clinker, the alkali-activated lithium slag composite cementitious materials significantly reduce CO₂ emissions and energy consumption.