In order to guide the gradation design of stone mastic asphalt (SMA) using steel slag, mixture design and vibrating compaction experiments of coarse aggregates were conducted to investigate the rules of voids in coarse aggregate (VCA) formed by different combination schemes of steel slag coarse aggregate (SSCA), basalt coarse aggregate (BCA), and hybrid coarse aggregate (HCA) of steel slag & basalt, respectively. Also, three regression models of VCA were established. The results show that there is a multiple nonlinear relationship between the VCA of SSCA and the proportion of each coarse aggregate in mixture, and the VCAs of BCA and HCA also have similar variation rules. For the same of aggregate gradation, the VCA value of SSCA is different from those of BCA and HCA. The size order of VCA and VMA values can be predicted using the regression model, which can provide basis for the determination of aggregate gradation in SMA proportion design.