Abstract: Globally distributed soft soils with high liquid limits can pose significant engineering challenges when used directly as foundation materials. A promising approach to address this issue is soil improvement using slag. To better understand the relationship between microstructural parameters and the strength of slag-modified soil, this study investigated the unconfined compressive strength of high-liquid-limit soft soil treated with different slag proportions and cured for 7 and 28 days. The soil microstructure was examined through scanning electron microscopy (SEM), and a corresponding discrete element model was established to analyze the mechanisms behind the mechanical property changes. The experimental results indicate that: (1) The optimal improvement effect was achieved with 40% slag content, reaching a maximum strength of 10 MPa.(2) As the reaction progressed, the microstructural parameters of the soil changed systematically, providing effective indicators for evaluating improvement effects.(3) The formation of geopolymers resulted in a denser force chain network, increased contact points, and enhanced contact strength under identical pressure conditions. Additionally, the soil structure maintained notable stability after compression.