Abstract: With the magnetized hot braised slag or unmagnetized hot braised slag, electric furnace slag and wind quenching slag as the research object, ethylene glycol, triethanolamine and absolute ethyl alcohol were used to prepare steel slag grinding aid. The steel slag ultrafine powders were obtained by mixing steel slag grinding aid and steel slag. The steel slag ultrafine powder/rubber composites were prepared by the steel slag ultrafine powder, carbon black N220, accelerant, sulfur, ZnO, stearic acid and the natural rubber. The influences of types of the steel slag and amount of steel slag grinding aid on the mechanical properties and flame retardant properties of the steel slag ultrafine powder/rubber composites were studied. The chemical composition, mineral composition, particle size distribution and composition structure were tested by XRF, XRD, LPSA and FTIR, respectively. The results show that the steel slag ultrafine powder/rubber composites prepared with electric furnace slag have good mechanical properties, the steel slag ultrafine powder/rubber composites prepared with electric furnace slag magnetized hot braised slag or unmagnetized hot braised slag have good flame retardant properties. The particle size of the steel slag ultrafine powder is reduced and the particle size distribution uniformity of the steel slag ultrafine powder is improved by steel slag grinding aid. With the increase of the amount of steel slag grinding aid, the particle size distribution uniformity of the steel slag ultrafine powder is improved, the mechanical properties of the steel slag ultrafine powder/rubber composites are improved and the flame retardant properties are reduced.