The mechanical properties， hydration products and hydration hardening processes of steel slag-slag based cementitious materials with different mix proportion were investigated using mortar strength tests and microscopic testing techniques such as X-ray diffraction （XRD）， thermogravimetric analysis （TG-DTG） and scanning electron microscopy-energy dispersive spectroscopy （SEM-EDS）. The results show that when n（CaO+ MgO）/n（SiO₂+Al₂O₃） is 0.90， more hydrated calcium silicate and hydrated calcium aluminate gels are generated in the later stage of hydration， the microstructure is more dense and the mechanical properties are the best. The compressive and flexural strength reach 20.20， 7.25 MPa at 28 days. The changes in pH value show the synergistic effect of hydration resulting from the dissolution of active minerals in steel slag and the occurrence of secondary fly ash reactions in slag. The optimal mix proportion of steel slag and slag ensures a higher level of hydration.