Nowadays, a large amount of steel slag (SS) not only occupies land resources but also pollutes the environment, and the low early strength and poor stability limit the application of SS in the building materials industry. In the study, SS was precarbonated and replaced 30% of cement to prepare a cementitious material. The optimum precarbonation conditions were determined. The hydration properties and microstructure of the precarbonated SS cementitious material were analyzed, and the strength and volume stability were studied. The results showed that the optimum precarbonation condition was carbonated at a water to powder mass ratio of 10% at 75°C for 2 h. After precarbonation, the early hydration rate and activity of the SS cementitious material had been improved. The calcium hydroxide [$\mathrm{Ca}{(\mathrm{OH})}_2$ ] of the precarbonated SS-cement paste no longer grew into a layered structure, and the total porosity of the paste decreased. The early flexural and compressive strengths of precarbonated SS cementitious material were greatly increased, and the volume stability was improved.