Steel slag particles with high hardness, high modulus and certain cementitious activity can be used as solidified soil material. The influence of steel slag on soil mechanics, stability and solidification mechanism are not clear. In this paper, the effects of steel slag content and steel slag particle size on soil California Bearing Ratio (CBR) value, water absorption, expansion rate and compressive strength were studied. By calculating the comprehensive, physical and chemical effects index of steel slag on soil modification, the solidification mechanism of steel slag on soil was described. In addition, XRD, IR and SEM - EDS were used to reveal the mechanism of steel slag on soil chemical modification. The results show that steel slag have significant physical and chemical modification effects on soil, and the mechanical and stability properties of soil increase with the increase of steel slag content, while decrease with the increase of steel slag particle size. When steel slag content is 8%, CBR value and 4d unconfined compressive strength (UCS) increase by 1014.4% and 180.0%, respectively, water absorption and expansion rate decrease by 53.0% and 45.4%, respectively, compare with that of plain soil. The decoupling analysis shows that the physical modification effect of steel slag is much weaker than the chemical modification effect, and the chemical modification effect plays a dominant role in the comprehensive modification effect of steel slag. The 4 d contribution weight of 0-0.6 mm steel slag to chemical modification is 72.2%, while the contribution weight of physical modification is only 27.8%. In addition, with the increase of age, the contribution weight of chemical modification is further increased. At 28 d and 90 d, the contribution weights of 0-0.6 mm steel slag to chemical modification increase to 78.7% and 86.3%, respectively. Microcosmic test results show that hydration and ion exchange reactions occurred after steel slag is added to soil. The C—S—H peak can be found in the XRD test results, and the enhanced vibration peak of the Al3+ cation coordination can be observed in the IR test results. From SEM test results, it can be seen that the floccule C—S—H gelatinous material has significantly fewer pores than plain soil, and its structure becomes more dense, which is consistent with the macroscopic mechanical properties.