Abstract: Modified steel slag powder was used to create a modified steel slag/rubber composite material using self-made steel slag grinding modifier and combining it with carbon black and rubber matrix to treat hot braised steel slag, electric furnace steel slag, and air-quenched steel slag. Next, the thermal conductivity of the three types of modified steel slag/rubber composites was measured using a thermal conductivity instrument at 1, 3, 5, 7, 9, and 11 days. The surface contact angle θ and crosslinking density of the above composites were calculated using Young’s and Flory’s equations before and after thermal oxygen aging, and their changes were analyzed using thermogravimetric analysis (TGA) and scanning electron microscopy (SEM). As a result, the thermal conductivity of the modified electric furnace slag/rubber composite material was the lowest 0.187 W·m⁻¹·K⁻¹. Among them, the median diameter (d₅₀) of the modified electric furnace slag particles was the smallest (3.49 µm) without thermal oxygen aging, easily forming a compact structure of rubber-wrapped slag, but more challenging to develop thermal conductivity paths that reduced thermal conductivity. In the process of thermal oxygen aging, the structure of rubber-wrapped slag was destroyed. While the modified electric furnace slag/rubber composite material had large pores and the best dispersibility, which reduced interface thermal resistance and easily formed thermal conductivity paths, its thermal conductivity was the highest. After thermal oxygen aging, it was found that long cracks, deep holes, and increased roughness lying on rubber composite material surface increase the water absorption and decrease the contact angle. Besides, due to the largest particle size of the modified hot braised slag, oxygen is more likely to enter the rubber composite material under the heat action to react with the rubber molecular chain (double bond) to generate free radicals, thus raising molecular weight and growing crosslinking density. The modified air-quenched slag had the highest basicity (3.3), was detrimental to the vulcanization process, and was prone to forming an unstable carbon layer, resulting in more secondary combustion and a lower crosslinking density. Moreover, the mass fraction of residual material called carbon residue was only 1.02% at 800 ℃ and it had the worst durability after thermal oxygen aging.