The surface of grade Ⅲ fly ash was modified through calcination method by mixing carbide slag. The mineral composition and surface morphology of the modified grade Ⅲ fly ash were characterized by using X ray diffraction(XRD), scanning electron microscopy(SEM) and nan scratch technique, and the hydration property of the modified grade Ⅲ fly ash was analyzed. The morphology and mechanical properties of micro interface between unmodified or modified grade Ⅲ fly ash were compared. The results show that the β C2S phase is formed on the surface of the modified grade Ⅲ fly ash, hydration of the modified grade Ⅲ fly ash will generate C S H gel which improves the micro interface microstructure between grade Ⅲ fly ash and cement paste, the porosity of micro interface of blended cement paste is decreased, and the mechanical properties of micro interface of blended cement paste is improved.