Abstract: Lithium mica smelting slag (lithium slag), rich in SiO₂, Al₂O₃, and CaO, has the potential to be used as a cementitious material. In this study, lithium slag, fly ash, and S95 slag powder were used as raw materials for preparing geopolymers. A single-factor experimental method was adopted, with a strategy of first determining the dosage of fly ash and S95 slag in the reference group and the optimal preparation conditions before replacing them with lithium slag. This approach was used to investigate in depth the effects of the mass ratio of fly ash to S95 slag, the modulus of water glass, alkali equivalent, liquid-solid ratio, and lithium slag content on the performance of geopolymers. Through multi-scale analyses, including FTIR and SEM, the interaction mechanism between aluminosilicates in lithium slag and alkali activators was revealed. The results show that under the optimal conditions of a fly ash-to-slag mass ratio of 6:4, a water glass modulus of 1.4, an alkali equivalent of 7%, a liquid-solid ratio of 0.3, and a lithium slag content of 15%, the prepared geopolymer exhibits a 28-day compressive strength of 62.3 MPa, meeting the compressive strength standard requirement for 52.5R early-strength cement (28 days of strength ≥ 52.5 MPa). Combined with SEM image analysis of geopolymer specimens, it was found that lithium slag exhibits defects, including high SO₃ content, low alkali-active substance content, poor water absorption capacity, and large particle size. These defects lead to increased numbers and sizes of microcracks and reduced gel content in the geopolymer after lithium slag incorporation, thereby causing a decline in mechanical properties. The research results provide data to support understanding the effects of multiple factors on the mechanical properties of geopolymer specimens, the development of high-performance lithium slag-based geopolymers, the comprehensive utilizing lithium slag, and the diversification of solid waste-based cementitious materials.