Abstract: The effect of ultrafine circulating fluidized bed fly ash (UCFA) on physical and mechanical properties of cement containing different contents of circulating fluidized bed fly ash (CFA) is studied,through rheological property and mortar strength of fresh cement paste test and combined with MIP and SEM microscopic test method,etc.The results show that cement single-mixed CFA has a larger water demand and poor fluidity,and its strength decreases significantly.Compared with cement single-mixed CFA,the cement of which CFA is partially replaced with 5% (optimum content) UCFA has a corresponding less water demand,greater fluidity ,smaller yield stress(τ₀) and plastic viscosity(η) of cement paste,and the workability of paste is improved obviously.Strength of cement containing 30% fly ash increases by 6% at 3 days,when strength of cement containing 60% fly ash increases by 5% at 28 days,reaching 32.5 cement grade requirements.The incorporation of UCFA can promote cement containing CFA to generate more ettringite and C-S-H gel and optimize pore structure of hardened cement paste,and thus improve strength of cement.