KR desulfurization slag is a kind of waste residue produced by desulphurization process of hot iron. The synergistic preparation of cementitious materials with various solid wastes is an effective way to recycle the desulfurization slag. In this paper, KR desulfurization slag, granulated blast-furnace slag and desulfurization gypsum were used to prepare solid waste-based cementitious materials, and the effects of KR desulfurization slag content and granulated blast-furnace slag content on the mechanical properties of cementitious materials were studied, so as to optimize the ratio of raw materials. The hydration products and hydration characteristics of solid waste-based cementitious materials were studied by XRD, TG-DSC, IR, SEM-EDS and hydration heat test methods. The results show that the optimum ratio of solid waste-based cementitious materials is KR desulfurization slag 25% (mass fraction, the same below), granulated blast-furnace slag 60%, and desulfurized gypsum 15%. The 3 d, 28 d and 90 d compressive strength of cementitious materials reach 30.01 MPa, 49.47 MPa and 55.73 MPa, respectively. The early hydration heat release rate of solid waste-based cementitious materials is low, and the cumulative heat release in 3 d is only 37.9% of ordinary Portland cement (OPC). The main hydration products are needle-rod ettringite (AFt) and amorphous calcium silicate hydrate (C-S-H) gel. A large amount of Ca(OH)2 in KR desulphurized slag can excite the slag alkali at the early stage of hydration to disintegrate the glassy silicate, and react with desulphurized gypsum to promote the formation of AFt. In the later stage of hydration, the synergistic reaction of KR desulfurization slag, granulated blast-furnace slag and desulfurized gypsum makes the hydration products increase continuously, and the dense structure is formed by mutual cementation, which is conducive to the continuous growth of strength.