The objective of the present paper is to study the physiochemical properties of fly ash under oxy-fuel circulating fluidized bed (CFB) combustion mode. Tests were conducted in a 50 kW oxy-fuel CFB combustor under both air and oxy-fuel combustion. The analyses of the collected fly ash samples included particle size analysis, N-2 adsorption analysis, char carbon, inductively coupled plasma optical emission spectrometry (ICP-OES), and X-ray diffraction (XRD). It was found that the d,o, d(50), and d(90) of fly ash under oxy-fuel combustion mode were larger than that under air combustion mode and increased with the rise of the inlet oxygen concentration. The Brunauer Emmett Teller (BET) surface area, pore volume, and pore size of fly ash under air combustion are almost the same with that at inlet oxygen concentration of 30%. The element mass concentrations were obviously different between the two combustion modes, but the main mineral phase was insignificantly different. Moreover, the inlet oxygen concentration has no significant effect on the element mass distribution in the fly ash, except for the alkaline earth metal under oxy-fuel combustion.