Steel slag asphalt mixtures with four kinds of steel slag contents were prepared. The four point bending fatigue test was carried out to analyze the influence of steel slag content and strain level on the fatigue properties and to reveal the improvement mechanism of fatigue performance. Results show that the fatigue life of asphalt mixture first increases and then decreases with the increase of steel slag content, and the peak appears at steel slag content of 30%(by mass). The initial stiffness modulus and fatigue life of steel slag asphalt mixture significantly reduce with the increase of strain level, the fatigue damage evolution process can be divided into three stages: instability, balance and failure. The fatigue failure modulus attenuation ratio Sr/S50 is less than 50%, and gradually decreases with the increase of strain level. The fatigue crack growth mainly occurs at the mineral asphalt interface and is affected by the strain level. The chemical properties and surface structure of steel slag provide the interactive adhesion and embedded anchor effect for the interface phase structure. Steel slag can significantly improve the fatigue properties of asphalt mixture when used as coarse aggregate.