Abstract: Alkali residue-based lightweight soil offers advantages such as a simple production process, reduced cement consumption, and low cost. It not only addresses the shortage of traditional fillers in engineering but also utilizes large quantities of alkali residue, thereby minimizing environmental pollution and generating significant economic and environmental benefits. In this study, scanning electron microscopy and mercury intrusion porosimetry were employed to investigate the effects of solid content, water-solid ratio, curing age, and wet density on the microstructural characteristics of alkali residue-based lightweight soil. The results indicate that hydration products (e.g., C-S-H gel and AFt) play a major role in determining the compressive strength of the material. With increasing GGBS content and wet density, prolonged curing age, or as the water-solid ratio approaches the optimal value of 0.6, the pore size distribution becomes more uniform and pore wall defects are reduced. The pore sizes in the material are predominantly concentrated in the range of 1.0~100.0 μm. Furthermore, porosity decreases with higher wet density, increased GGBS content, and extended curing age, but increases with a higher water-solid ratio.