Abstract: In view of the significant differences in unstable factors such as free calcium oxide (f-CaO) in steel slag with different particle sizes, a combined pretreatment method involving sieving classification, mechanical grinding and steam digestion was adopted to modify steel slag of various particle sizes. The pretreated steel slag was then mixed with ground granulated blast furnace slag to prepare a steel slag–slag composite cementitious material. The effects of steel slag particle size and steam digestion time on the phase composition and mechanical properties of the composite system were investigated. The results show that steam digestion is effectively used to promote the conversion of f-CaO in steel slag into Ca(OH)₂ and CaCO₃, by which the phase stability of the steel slag is significantly improved. The cementitious performance of the composite system is found to be closely related to the particle size of the steel slag and the digestion time. After steam digestion for 1 hour, the mass fraction of f-CaO in the steel slag with a particle size of 0.15–0.30 mm is reduced to 1.20%, and the 28-day compressive strength of its composite cementitious material is measured as 28.25 MPa. However, for the steel slag powder with a particle size smaller than 0.15 mm, the mass fraction of f-CaO is only 0.77%, so it can be used directly without steam digestion. By means of a classification pretreatment and synergistic utilization strategy based on particle size differences, the performance of steel slag is optimized while energy consumption is reduced. In this study, a theoretical basis and technical ideas are provided for the high-value and large-scale utilization of steel slag.