Abstract: The modified slag powder (MSP) was obtained by surface treatment of the slag powder (SP) with the modifier Si−69, PP/MSP composites were prepared by melt blending using polypropylene (PP) as the matrix and MSP instead of traditional inorganic filler. The influence of MSP on the mechanical properties of PP/MSP composites was analyzed, and the distribution of MSP in PP matrix and the close binding degree of MSP to PP were discussed. The results show that with the increase of MSP content in the range of 0−40%, the tensile strength of PP/MSP composites increases first and then decreases but is higher than that of pure PP samples, the elastic modulus increases, and the impact strength increases first and then decreases. When the mass fraction of MSP is 20%, the comprehensive mechanical properties of PP/MSP composites are the best, the elastic modulus is 508.9 MPa, which is 39.6% higher than that of pure PP sample, and the impact strength is also increased from 1.83 kJ/m² to 1.96 kJ/m².One side of modifier Si−69 is long chain alkyl and the other side is hydrolysis generated Si—OH, Si—OH is bound to SP surface —OH. The affinity between long chain alkyl and PP matrix is strong, which makes MSP closely bound to PP matrix, greatly reduces the agglomeration of MSP in PP matrix, and greatly improves the mechanical properties of PP/MSP composites.