Currently, most steel slag is used as aggregate for concrete, asphaltic paving and road bases. High-alkalinity steel slag shows certain cementitious properties and should be given priority to be used as a supplementary cementitious material in concrete, based on economical and environmental considerations. This study was conducted to propose an effective method to evaluate the activity of high-alkalinity steel slag. Although the alkalinity determines the types of major minerals of steel slag, it does not determine the contents of these minerals. The difference of activity among different types of high-alkalinity steel slag cannot be further distinguished by alkalinity. The activity index (AI = (SiO2 + Al2O3)/(FeO + Fe2O3 + MgO + MnO)) was defined to evaluate the hydration activity of steel slag, which was verified to be effective by the results of examples. The higher the activity index, the higher the hydration activity of the steel slag.