To explore the influence of steel slag mineral compositions on the performance of asphalt mixtures, this study conducted water stability tests on asphalt mixtures using different types of steel slag with 0%, 30%, 60%, and 100% blending ratios, as well as mixtures containing 30% K steel slag before and after aging, and conducted mineral composition analysis using XRD. The results showed that compared with ordinary limestone powder and cement, the content of silicate minerals and active minerals such as CaO in different types of steel slag varied greatly, and there were significant differences in the improvement of asphalt mixture performance with an increase in Marshall stability and splitting strength in the range of 28.0% to 40.0%. With the increasing amount of K steel slag, the content of active components increased, leading to improved performances of the asphalt mixture with an increase in the Marshall stability and splitting strength by 38.0% to 40.0%. After aging, the content of active minerals in steel slag decreased, and the improvement effect on the asphalt mixture weakened. Moreover, the water stability decreased with the increase of active components in the steel slag. The incorporation of 30% K steel slag resulted insignificant improvement in the water stability and splitting strength of the asphalt mixtures.