The active Si/Al ratios of metakaolin-based geopolymer raw materials are closely related to their mechanical properties. In this study, the content of active Si and active Al in metakaolin were determined by acid leaching-alkali dissolution method and the complexometric titration method. By adding active Si and active Al and employing characterization techniques such as XRD, FTIR, DSC-TG, and SEM, the effects of different active Si/Al ratios on the geopolymerization process and mechanical properties of metakaolin-based geopolymers were investigated from the perspectives of material composition and microstructure. The results show that the acid leaching-alkali dissolution method and the complexometric titration method can effectively quantitatively analyze the contents of active Al and active Si in metakaolin. With the increase of active Si/Al ratio, the 3 d compressive strength of the metakaolin-based geopolymer initially increases and then decreases. When the active Si/Al ratio is relatively low (0.4~0.8), the “encapsulation effect” in the metakaolin-based geopolymer system leads to an increase in unreacted aluminosilicates, and the structure with numerous pores and cracks, resulting in low compressive strength. When the Si/Al ratio is relatively high (2.2~2.4), the system forms zeolite structure and becomes a multiphase system, and the compressive strength is also low. When the active Si/Al ratio is 1.0, the degree of geopolymerization reaction is maximized, yielding the highest amount of gel phase and the best compressive strength, with a compressive strength of 33.7 MPa. These findings provide a theoretical basis for the regulation of the compressive strength in metakaolin-based geopolymers.