Hydration mechanism of cementitious composite based on high-titanium slag and cement was studied by micromorphology of hardened paste using SEM, XRD and TG-DSC. The results show as follows：High-titanium slag, whose hydration activity is quite low, is mainly composed of stable minerals with strong crystallinity. High-titanium slag particles can disperse cement particles as well as fill in between them to improve paste structure obviously. In the early period of hydration reaction, the hardened paste structure is loose and the hydration products are relatively less, and there still exist lots of unreacted stable minerals. In the late period of reaction, there is a secondary reaction of hydration between high-titanium slag and Ca(OH)₂, resulting in stable growth of mortar strength