Steel slag tailings are the main solid waste in the metallurgical industry, with the production of 15%~20% of crude steel. Due to limited technology, the utilization ratio is quite low and only reaches 10% of steel slag tailings production. Meanwhile, steel slag tailings are disposed of in direct stacking and landfill since the management system is not perfect, which pollutes land, underground water, and air quality. In the face of the above problems, in this study, the modified steel slag powder was developed by hot slag, electric furnace slag and air quenching slag, and the modified steel slag powder was compounded with composite rubber to prepare modified steel slag/rubber composites. According to the “Accelerated aging and heat resistance test of vulcanized rubber or thermoplastic rubber in hot air” (GB/T3512—2014), the modified steel slag/rubber composites were subjected to thermo-oxidative aging treatment. The cross-linking density of the modified steel slag/rubber composites was determined by the equilibrium swelling method. The microstructure, weight loss rate and structural composition of the modified steel slag/rubber composites were tested by scanning electron microscopy (SEM), thermogravimetric analyzer (TGA), and Fourier transform infrared spectrometer (FTIR), respectively. The thermo-oxidative aging mechanism of the modified steel slag/rubber composites was expounded from the micro level. The results show that the cross-linked bond formation reaction is the main reaction in the surface of modified steel slag/rubber composites during the early thermal-oxidative aging. In the middle stage of thermo-oxidative aging, the aging effect has acted on the internal of modified steel slag/rubber composites, resulting in the fracture reaction rate of cross-linking bonds higher than that of forming a large number of broken cross-linking bonds. In the later stage of thermal-oxidative aging, many broken cross-linked bonds have been formed in the modified steel slag/rubber composites, resulting in the decrease of the fracture speed of the main chain and cross-linked bonds and the formation of cross-linked bonds is dominant. The modified steel slag powder with high SiO2 content as raw material is beneficial to form the structure of polymer macromolecular chain through carbon black network and improves the comprehensive performance, especially the physical and mechanical properties and hysteresis. Preparation of modified steel slag powder with electric furnace slag and air quenching slag (high Fe2O3 content) is beneficial to improving heat conduction performance.