In Oxy?fuel circulating fluidized bed, the residual CaO particles may react with high concentration of CO2 in flue gas to form bonded deposit on heat transfer surfaces in backpass when limestone is used as a sorbent to capture SO2 .In this paper, experiments were designed on ash deposition in a bench?scale fluidized bed under oxy?fuel and air atmosphere. A novel ash deposit sampling probe was used to simulate the tubes of tail surfaces. The chemical composition of fly ash and ash deposit from both air?firing and oxy?fuel firing cases were analyzed by Inductively Coupled Plasma?Atomic Emission Spectrometry ( ICP?AES ) and Scanning Electron Microscopy ( SEM) , respectively. The degrees of carbonation reaction of ash deposits were measured by Thermo Gravimetric Analysis. The results showed that there are distinct differences in fly ash deposition rate between oxy?fuel and air firing cases, and oxy?fuel combustion with limestone addition can affect chemical composition of fly ash and ash deposit, especially for elements of Ca, Na, K, and S. However, the carbonation reaction degree of ash deposits is found weak, which is due to the relatively low CaO content in ash deposit or not long enough of the sampling time.