Abstract: To further improve the comprehensive performance of steel slag mixed soil, polypropylene fibers with good toughness were incorporated into the mix to prepare fiber-reinforced steel slag soil. The effects of fiber length and dosage on the unconfined compressive strength, dry shrinkage, freeze-thaw resistance and other properties of the steel slag mixed soil were studied. Scanning electron microscope was used to characterize and analyze the effect of reinforcement on the microstructure of frozen specimen, and the mechanism of fiber reinforcement on the comprehensive performance of steel slag mixed soil was investigate.The results show that fiber reinforcement can effectively improve the compressive, dry shrinkage and frost resistance of steel slag mixed soil.The optimal comprehensive performance is achieved with fiber length of 12 mm and a dosage of 0.25%. The ductility of the reinforced specimen improves, with compressive strength increasing by up to 16.74%.While the water loss increases, the dry shrinkage coefficient decreases by up to 37.51%. After freezing, the mass loss rate decreases, and the residual strength ratio increases by up to 6.33%. At this stage, the microstructure of the specimens is relatively intact, thus exhibiting better dry shrinkage and freeze-thaw resistance. The effective bonding between polypropylene fibers and the matrix provides interfacial friction and formation of a three-dimensional network structure within the matrix is a critical basis for the enhanced performance of the steel slag mixed soil.