To advance China’s “dual-carbon” strategy and address the high-carbon bottleneck of cement-only binders in flowable solidified soil (FSS), this study proposes, for the first time, a novel cementitious system in which ground granulated blast-furnace slag (GBFS) and steel slag (SS) jointly and partially replace cement for FSS stabilization. Flow spread (slump flow), setting time (initial and final), unconfined compressive strength (UCS), and microstructural characteristics (XRD, SEM) were tested to systematically elucidate the synergistic mechanism of GBFS and SS on FSS performance. The results show that decreasing the SS proportion reduces the flow spread (216～164 mm) and accelerates its rate of decline, while increasing GBFS markedly shortens both initial and final setting times. Across all mixtures, initial and final setting times ranged from 189～215 min and 385～410 min, respectively. Compressive strength was strongly governed by the GBFS-SS proportions; at a SS:GBFS:cement mass ratio of 1:4:5, the 28-day UCS peaked at 3.05 MPa, a 23% increase over the single-admixture system (S50-G0). Microstructural analyses indicate that this mixture exhibited the most complete pozzolanic reaction, the lowest porosity, and a greater abundance of hydration products (C—S—H, C—A—H, AFt), forming a dense interlocked microstructure. These findings demonstrate that jointly and partially replacing cement with GBFS and SS effectively optimizes the performance of FSS.