Abstract: In order to make full use of the ground circulating fluidized bed combustion slag (CFB), reactive MgO, CFB and silica fume (SF) were used to prepare MgO-CFB-SF composites. The good fluidity, high strength and low cost MgO-CFB-SF composites were gained. The effects of curing temperature, SF and MgO dosage on the strength, reaction products and microstructures of MgO-CFB-SF were investigated. The effect of MgO's activity and fineness on compressive strength of MgO-CFB-SF was preliminarily studied. The yield stress and plastic viscosity of the MgO-CFB -SF paste were studied by rheometer. The reaction products and microstructure of MgO-CFB-SF with different ages were analyzed by XRD, TGA, FTIR and SEM. It is found the mixing of SF helps to improve the strength of the MgO-CFB-SF composites, and the composite paste with 16wt%-20wt% SF and 20wt% MgO has good flowability. The microanalysis results of MgO-CFB-SF samples maintained for 540 days at room temperature show that the main phase is magnesium silicate hydration(M-S-H), and hydrotalcite-like phase (Ht) or Al-rich magnesium silicate hydration (M-S(A)-H).