Abstract: In order to realize the high-value utilization of metallurgical solid waste steel slag and enhance the comprehensive performance of stone-wood plastic composites. Using homemade grinding aids and steel slag composite for ultrafine vertical milling, and silane coupling agent for its modification, produced modified steel slag superfine powder (MSSP), partially replacing talcum powder used in the preparation of stone-wood plastic composites. The effect of MSSP on the mechanical and flame retardant properties of stone-wood plastic composites and its mechanism were studied. The results showed that when MSSP replaced 50 % of talc, the stone-wood plastic composites obtained the best mechanical properties and flame retardant effect, and compared with pure talc/stone-wood plastic composites, its tensile strength, flexural strength and impact strength increased by 55.5 %, 36.3 % and 76.7 %, respectively; the horizontal combustion speed and vertical combustion after-flame time and after-scorching time were significantly reduced, and the oxygen index increased by 22.8%. Through the modification of silane coupling agent, the compatibility between MSSP and stone-wood plastic organic interface was effectively improved; MSSP was uniformly distributed and grafted to the molecular chain of polyethylene and wood powder to enhance the interaction with the matrix, and the high hardness and high melting point of MSSP promoted the stress transfer within the stone-wood plastic system, which strengthened the mechanical properties of the stone-wood plastic composite material; the residues with polyethylene and wood powder in the combustion process rapidly coalesced to form a dense and stable protection. During the combustion process and the residue of polyethylene, wood powder, etc., a dense and stable protective charcoal layer is formed quickly, which further enhances the flame retardant property of the material and realizes the dual effect of reinforcement - flame retardant. This study opens up a new way for the high-value utilization of steel slag and contributes to the promotion of resource recycling and sustainable development.