Abstract: High-temperature modification by iron tailings was performed to study its influence on the volume stability and cementitious properties of steel slag. The mineral constituents and microstructure of steel slag after the high-temperature modification were analyzed via X-ray diffraction (XRD), scanning electronic microscopy (SEM) and energy dispersive spectrometry (EDS). It is found that the content of f-CaO obviously decreases and the cementitious properties of steel slag improve through the high-temperature modification. Compared with raw steel slag, the mass fraction of f-CaO in steel slag decreases from 4.84% to 1.82% after modification with 20% iron tailings at 1 250℃ and the activity index increases up to 5.6% after 28 d. With an increase in the mass fraction of iron tailings from 10% to 30%, silicate mineral phases such as manganolite, akermanite and calcium magnesium pyroxene form in the modified steel slag. The high-temperature modification promotes the decomposition of RO phase, in which FeO transforms to magnetite (Fe₃O₄) simultaneously.