Circulating fluidized bed fly ash (CFBA) was used to replace pulverized fuel ash (PFA). The effects of different CFBA substitution amount and cement content on compressive strength and dry density of autoclaved aerated concrete (AAC) were studied. Then, chemical titration, X-ray diffraction (XRD) and scanning electron microscope (SEM) were used to explore the characteristics of delayed formation of ettringite (AFt) in AAC. The results show that when CFBA replaces 40% (mass fraction) PFA and cement content is 20.0% (mass fraction), AAC meeting the requirements of dry density grade B05 and strength grade A3.5 is prepared. When CFBA replaces 40% PFA and 100% PFA, the AFt content in 7 d air cured sample without autoclaved treatment reaches the maximum value of 7.71% (mass fraction) and 18.60%, respectively. After AAC obtained by autoclaved treatment and then air curing for 28 d, the amount of AFt delayed in sample is less than 1.00%. After AAC prepared by autoclaved treatment, and then soaked in water for 28 d, the AFt production in sample with CFBA replacing 40% PFA is 2.57%, and the AFt production in sample with CFBA replacing 100% PFA is 4.46%. The delayed AFt formed in AAC sample with CFBA replacing 100% PFA is a typical needle-rod crystal with high aspect ratio, which has the risk of volume stability.