[Purposes] Expansive soil is a type of clay with high dispersibility and high plasticity. It swells upon absorbing water, disintegrates or softens when exposed to water, shrinks when losing water, has poor anti-erosion performance, and is extremely sensitive to changes in dry and wet conditions. It is a "special soil with many problems." [Methods] This paper aims to study the mechanical and expansive properties of carbide slag (CS) + ground granulated blast-furnace slag (GGBS)-stabilized expansive soil. Through experiments and theoretical analysis, the micro-mechanism is revealed, providing new methods and theoretical basis for the improvement of expansive soil. [Findings] The experimental results show that when the mixing ratio of the stabilizer is 15% and the ratio of CS to GGBS is 2:8, the swelling rate is the lowest, at 0.029%, while the unconfined compressive strength is the highest at both 14 days and 28 days of curing, reaching 7.31 MPa and 7.97 MPa, respectively. Microscopic tests conducted through SEM and XRD analysis reveal that after the reaction between CS and GGBS, calcium silicate hydrate(C-S-H) gel, ettringite, and other cementitious products are formed, binding soil particles and forming a dense structure with loose soil particles, thereby improving the mechanical properties of the soil. [Conclusions] The CS-GGBS composite system can effectively improve the swelling characteristics and mechanical properties of expansive soil, with the optimal ratio of CS:GGBS=2:8 at a mixing content of 15%. Its improvement mechanism lies in that the cementitious products fill the soil pores and bond soil particles, thereby enhancing the soil strength and reducing soil swelling.