Abstract: Aiming at the problem of low early strength of steel slag-desulfurized ash-based cementitious materials, the effects of different early strength agents on the compressive strength and hydration performance of steel slag-desulfurized ash-based cementitious materials were studied. Experimental research results showed that when the desulfurization ash content was 15%, the 3-day compressive strength was 3.65 MPa, which was 48.95% lower than the blank sample. Using quicklime, calcium chloride, calcium formate, silica fume, aluminum sulfate and other composite early-strength agents to modify steel slag-desulfurization ash-based cementitious materials, the 3-day compressive strength reached 17.89 MPa, which was 4.90 times that of the blank sample, and met the cement compressive strength requirements of composite Portland cement 42.5 grade, specified in China’s national standard, GB 175—2007. Through XRD, SEM, FI-RT, XPS and DTG tests, it was found that the main hydration products of cementitious materials were monosulfur hydrated calcium aluminum sulfide, hydrated calcium chloroaluminate, and hydrated calcium silicate, and Friedel’s salt.