Due to the low early hydration activity, poor grindability and soundness of steel slag, which restricts its large-scale utilization in cement and concrete. In this paper, the mineral phase transformation and the changes of grindability, soundness and activity index of reconstructed steel slag at different reconstructed temperatures were studied. The results show that the high temperature optimizes the mineral phase composition of steel slag, promotes the transformation of the refractory phase (FexO) and RO phase, promotes the transformation of dicalcium silicate (C2S) to tricalcium silicate (C3S) in steel slag, and promotes the formation of magnesium iron spinel (MgFe2O4). The reconstructed steel slag with uniform mineral and liquid phase distribution, good mineral composition and clearer boundary often shows higher strength, and the 28 d activity indexes of the two kinds of steel slag at the 1 400 ℃ reach 99.03% and 96.52%, respectively. The f-CaO content in steel slag decreases significantly with the increase of reconstructed temperature, and the grindability increases first and then decreases with the increase of reconstructed temperature.