Abstract: Ultra-fine fly ash was utilized as the admixture of concrete by non-autoclaved curing. The fluidity and mechanical properties of concrete were investigated with different dosages of ultra-fine fly ash. The results show that the slump and demoulding strength of concrete are in a positive correlation with the dosage of ultra-fine fly ash lower than 16%. The slump and demoulding strength rarely increase or even decrease a little when the dosage exceeding 16%. The tensile/compressive strength ratio increases with fly ash due to a reduction in brittleness. The demoulding strength and 28d strength of C80PHC concrete with a slump of 150mm reach to 79.4MPa and 87.4MPa respectively after a steaming curing(uniform raising temperature for 2h, constant temperature of 85℃ for 5h, lowering temperature for 1h).