Abstract: To address the issues of insufficient cementitious activity in steel slag-based cementitious materials caused by high-content inert mineral phases and the high energy consumption of conventional thermal activation processes, this study developed a hydrothermal synthesis-low-temperature calcination method combined with alkali activation to prepare steel slag-based cementitious materials, aiming to dissociate inert components in steel slag and activate their reactivity at low temperatures for efficient resource utilization. By introducing NaOH and Na₂SiO₃ activators, along with hydrothermal synthesis and low-temperature calcination, the effects of calcium-to-silicon (Ca/Si) ratio, activator type, and concentration on the phase composition of hydrothermal precursors and the reactivity of calcined products were systematically investigated using X-ray diffraction (XRD), thermogravimetric-differential thermal analysis (TG-DTG), and scanning electron microscopy (SEM). The results demonstrated that increasing the Ca/Si ratio to 1.5 with 12% Na₂SiO₃ (calculated as Na₂O) disrupted the crystalline structure of inert phases in steel slag under combined alkaline and thermal effects, significantly enhancing the formation of calcium silicate hydrate (C−S−H) gels during hydrothermal reactions. These gels were subsequently converted into the reactive β-C₂S phase after calcination at 700°C, thereby improving the hydration performance of the cementitious material. Compared to untreated steel slag (7-day and 28-day compressive strengths of 0.54 MPa and 1.02 MPa, respectively), the material with 90% steel slag incorporation exhibited 7-day and 28-day strengths of 0.99 MPa and 2.93 MPa, representing increases of 83.7% and 194.6%. The findings confirm the feasibility of dissociating inert mineral phases in steel slag via alkali activation coupled with hydrothermal synthesis to produce calcium silicate hydrate, followed by low-temperature calcination for cementitious material preparation, demonstrating potential for large-scale consumption of waste steel slag in mine backfill applications.