Abstract: Metakaolin-slag geopolymer concrete, as a green high-performance material, exhibits mechanical properties that are highly sensitive to the slag content. This paper investigates the influence mechanism of slag content on the basic mechanical properties of metakaolin-based geopolymer concrete using a multi-scale research approach (covering macro, meso, and micro levels) through mechanical performance tests, microstructure analysis, and molecular dynamics simulations. The experimental results show that the basic mechanical properties of geopolymer concrete initially increase and then decrease with increasing slag content. When the slag content is 30%, the compressive strength, flexural strength, and dynamic elastic modulus of the geopolymer reach their peak values of 55.8 MPa, 4.6 MPa, and 72.63 GPa, respectively, indicating a more complete precursor reaction and improved microstructural density. Five C–(N)–A–S–H gel models with different slag contents were established using Materials Studio. Simulation results show that the diffusion coefficient of Al is smaller than that of Si, and in the geopolymer model with 30% slag content, the bond lengths of key chemical bonds (such as Ca–O、Al–O、Si–O)are closer to their ideal coordination values, confirming its optimal mechanical performance.