To investigate the rheological properties of steel slag unclassified tailings slurry under different mixing ratios, rheological tests were conducted using the controlled shear rate method. The effects of mass concentration, cement-sand ratio, and steel slag content on the rheological parameters of the slurry were analyzed, and their rheological properties were elucidated. The interaction between mass concentration, cement-sand ratio, and steel slag content on the rheological parameters of the slurry was explored using the central composite design response surface method. The results show that the influence of mass concentration on yield stress and viscosity is most significant. With the increase of mass concentration, yield stress and viscosity follow a power function growth law. The addition of steel slag reduces the viscosity and yield stress of the slurry, significantly improving its fluidity. When the steel slag content increases from 0% to 25%, the yield stress decreases by 85.43% and the viscosity decreases by 53.83%. For the interaction, the interaction between mass concentration and steel slag content on yield stress and viscosity is most significant. The increase of steel slag content promotes the degree of influence of mass concentration on rheological parameters, that is, with the increase of steel slag content, the influence of mass concentration on yield stress and viscosity gradually increases. Parameter optimization shows that when the mass concentration is 60%, the steel slag content is 30%, and the cement-sand ratio is 1:8, the yield stress and viscosity of the slurry are the lowest, and the flowability is the highest.