Abstract: To further improve the comprehensive performance of steel slag mixed soil, in the steel slag mixed soil with good toughness of polypropylene fibers, the preparation of fiber reinforced steel slag mixed soil, the study of fiber length, dosage on the steel slag mixed soil unconfined compressive strength, dry shrinkage, freezing, and other properties, and the use of scanning electron microscope characterization analysis of the effect of reinforcement on the frozen specimen microscopic morphology. The results show that fiber reinforcement can effectively improve the compressive, dry shrinkage, and frost resistance of steel slag mixed soil, and the comprehensive performance is relatively optimal when the length of reinforcing fiber is 12 mm and the dosage is 0.25%. The ductility of the reinforced specimen becomes better, and the compressive strength increases up to 16.74%; the water loss increases, but the dry shrinkage coefficient decreases up to 37.51%; the mass loss rate decreases after freezing, and the residual strength ratio improves up to 6.33%, and its micro-morphology is relatively intact, so it exhibits better resistance to dry shrinkage and freezing performance. The interfacial friction provided by the effective bonding between polypropylene fibers and the matrix and the three-dimensional mesh structure formed in the matrix is the important basis for the performance enhancement of the steel slag mixed soil.