The chemical composition of circulating fluidized bed fly ash (CFBFA) are significantly affected its hydrated cementitious properties. Based on the effects of composition and content on the hydrated cementitious properties for CFBFA, it is helpful to further clarify the hydrated cementitious mechanism of CFBFA. The effects of composition and content on the hydrated cementitious mechanical properties, phase composition, micro morphology and elemental distribution for CFBFA were investigated by mechanical properties testing, X-ray diffraction analysis and scanning electron microscopy-energy spectroscopy analysis. The results show that the hydrated cementitious properties are obviously different for CFBFA mortar test block with the different chemical compositions. When the mass contents of aluminum and silicon exceeds 75%, the 28 d compressive strength of CFBFA mortar test block is about 38 MPa. When the mass content of calcium and sulfur exceeds 28%, the 28 d compressive strength of CFBFA mortar test block decreases to about 14.5 MPa. The high content of aluminum and silicon are beneficial to the formation of dense blocks with the large percentage of hydrated calcium silicate, while the high content of calcium and sulfur will result in the appearance of rod-shaped calcium alumina, which is not conducive to the development of mechanical property. In addition, with the increase of cement mass fraction from 50% to 90%, the 28 d compressive strength of CFBFA mortar test block increases by 27.2 MPa. The compressive strength of CFBFA mortar test block is positively correlated with the additional amount of cement in hydrated cementitious system, due to the hydration effect of cement itself and its promoting effect on the reaction of CFBFA. This study can provide a theoretical guidance for the application of CFBFA in the field of construction materials.