In view of the problem that steel slag is prone to expand in base material, steel slag, blast furnace slag micro-powder and soil were mixed to prepare a road base material-steel slag mixed soil using soil solidification technology. The compaction, expansion rate and unconfined compressive strength tests were carried out on 16 soil samples with different mixture ratios. The results show that the optimum moisture content of the steel slag mixed soil is little affected by the mixture ratio, which maintained at 12~14%. The maximal dry density decreases with steel slag content, and decreases as the content of blast furnace slag micro-powder rises. Blast furnace slag micro-powder could significantly reduce the expansion rate of steel slag mixed soil and increase its unconfined compressive strength. The 7 d, 14 d, 28 d and 90 d unconfined compressive strength of steel slag mixed soil are firstly increase then decrease with steel slag content, and the strength reaches the maximum value when the mixture ratio is 50% steel slag and 50% soil. Microstructural analysis shows that the change of surface charge of soil particle enhance the compactness of steel slag mixed soil in molding process. The hydration reaction of f-CaO in the steel slag was inhibited during solidification, and which conduct digestion reaction with SiO₂ in the mineral slag to form hydrated calcium silicate gel (C—S—H). At the same time, inter-particle stacking effect between steel slag and soil and self-compacting effect produced by C—S—H gel could enhance strength of steel slag mixed soil, and thus steel slag mixed soil with high strength and stability is prepared.