Abstract: The functional material (called shield powder) was formed by mixing functional additives with ordinary carbon steel slag under the work of ultra-fine vertical mill, and replaced the flame retardant filler aluminum hydroxide to form shield powder/rubber composite material. In order to reveal the flame retardant mechanism of shield powder in the rubber system, the paper tested vulcanization properties, mechanical properties and combustion properties and analyzed the gas phase and solid phase residues in the combustion process of shield powder/rubber composite material. The results show that, the shield powder prepared from steel slag could promote the vulcanization process of rubber system, shorten vulcanization time and increase vulcanization rate index. What is more, it can replace aluminum hydroxide as flame retardant filler of rubber system and has little effect on mechanical pro-perties. Besides in the combustion process of shield powder/rubber flame retardant composite material, there are Al₂O₃, MgO, SiO₂, Fe₂O₃ and other substances in shield powder to form a synergistic flame retardant-smoke extinguishing system. Furthermore, the main mineral composition of carbon slag of shield powder/rubber flame retardant composite material is ZnS and FeS₂ and is closely related to the proportion of shield powder replacing alumina hydroxide. With the increase of substitution ratio, there are SiO₂ and MnP newly in carbon slag.