In order to study the resource utilization of slag, fly ash and carbide slag, the hydration product composition and strength characteristics of slag-fly ash composite cementitious materials were studied with carbide slag as alkali activator. The crystal structure, thermochemical properties and microscopic morphology of the composite cementitious materials were analyzed by X-ray diffraction (XRD), fourier transform infrared spectroscopy (FTIR), thermogravimetric-differential scanning calorimetry (TG-DSC), scanning electronic microscopy-energy dispersive spectroscopy (SEM-EDS), and the interaction mechanism of carbide slag activating slag-fly ash composite cementitious materials was studied. The results show that carbide slag as alkali activator provides a strong alkaline environment for the initial hydration of slag-fly ash composite cementitious materials, and it can also drive the hydration reaction of slag and fly ash. With the increase of slag content, the strength of composite cementitious materials increases first and then decreases. When the mass proportion of fly ash and slag is 4∶6, and the mass addition of carbide slag is 4%, the compressive strength of composite slurry reaches 25.9 MPa under the conditions of 4 d normal temperature curing and 32 h high temperature steam curing. The hydration products of the slag-fly ash composite cementitious system are unevenly distributed, containing C-S-H, C-A-H, C-A-S-H gel and etc. Carbide slag has a good effect as alkali activator in slag-fly ash system.