In order to promote the application of small-size steel slag in thin-layer asphalt mixtures, the aggregate mix ratio of SMA-5 asphalt mixtures was designed using the V-S (volumetric mix) method. Adding 1.18~2.36 mm and 2.36~4.75 mm steel slag aggregates (equal volume replacement of limestone aggregates) into SMA-5 asphalt mixtures, and its effect on the percent voids in coarse mineral aggregate VCAmix in asphalt mixtures and the optimum asphalt content of asphalt mixtures was analyzed. The high temperature stability, low temperature cracking resistance and moisture stability of steel slag asphalt mixtures were evaluated using rutting test, beam bending test, freeze-thaw splitting test and water-immersion Marshall test. The results show that the inclusion of steel slag reduces the effect of skeleton structure between aggregates and increases the amount of asphalt used in asphalt mixture. The high temperature stability of asphalt mixture is reduced by adding steel slag. However, with the increase of steel slag content, the low temperature performance of asphalt mixture decreases, and the moisture stability and high temperature stability increase. It is recommended that 75% of steel slag of 2.36~4.75 mm in SMA-5 steel slag asphalt mixture is the optimal ratio.