To solve the problem of difficult utilization of steel slag, the liquid steel slag was modified and the air-quenching granulation process was carried out to make steel slag into a value-added end product: air-quenching granulated steel slag. The granulated slag was tested to analyze the variation rule of slag properties under different modification conditions. Based on the phase diagram of CaO–Si₂O–FeO–MgO–Al₂O₃ slag system, the feasibility of blast furnace (BF) slag as modifier was determined. When the addition of BF slag was increased from 0% to 35%, following results were obtained. The slag fluidity was improved, and the air-quenching temperature range was expanded. Then, the yield of air-quenched steel slag increased, while the granulation rate, the degree of sphericity, the compactness were decreased. Furthermore, the air-quenching granulation process could substantially improve the stability and the amorphous content of steel slag. The maximum removal rate of free CaO was above 80% and the amorphous content was up to 95%. Taking the factors of yield and properties of granulated steel slag into full consideration, the optimum proportion of BF slag is around 15%.