Surface modification processes significantly influence the aggregate characteristics of steel slag and the road performance of asphalt mixtures. Therefore, this study employs four different substances to conduct surface modification treatment on aged steel slag with particle size ranges of 4.75–9.5 mm and 9.5–13.2 mm to determine the optimal surface modification process through macroscopic and microscopic analytical methods. Ultimately, a comparative analysis is performed on the pavement performance of asphalt mixtures incorporating aged steel slag and modified aged steel slag at different volume replacement ratios. Experimental results demonstrate that Epoxy Acrylate-Modified Organosilicon Resin (EAOR) significantly improved the aggregate properties of Aged Steel Slag (ASS), making it a viable alternative to natural stone; under identical conditions, the volume stability of modified aged steel slag (EAOR-ASS) was slightly superior to that of ASS, while the differences in high-temperature stability and low-temperature cracking resistance of the corresponding asphalt mixtures were minimal. Surface modification of ASS with EAOR significantly enhanced the water damage resistance of the asphalt mixture under freeze–thaw cycle conditions, thereby improving the utilization rate of steel slag. Both an increased volume replacement ratio of steel slag and surface modification with EAOR contributed to improved resistance to elastic deformation of the asphalt mixture.