In order to promote large quantity and high value-added utilization of steel slag mud, the hydration and hardening characteristics of steel slag mud from Qian’an of Hebei under blast furnace slag and desulphurization gypsum were investigated with XRD, FE-SEM, FT-IR and TG-DTA. The results indicate that the steel slag mud still shows good hydrated cementitious property after mechanical grinding. Compared with ordinary steel slag-blast furnace slag-desulphurization, the compressive strength of cementitious materials has the advantage of high early strength. Its hydration products are mainly AFt and C-S-H gel. During the hydration reaction process, the steel slag mud provides an alkaline environment for the system and promotes the dissociation of the vitreous in the slag. The slag hydration continuously consumes hydroxyl groups to further promote the hydration of the steel slag mud. The desulfurized gypsum provides a large amount of Ca2+and SO2-4, which react with gel in the system to produce AFt. The three materials interact with each other to synergistic reaction and promote the continuous hydration reaction.