Combustion experiments of semianthracite coal were conducted on a 6 kW(th) oxy-fuel circulating fluidized bed combustor. The inner relationship between CO, NO, and Hg in the combustor under an O-2/CO2 atmosphere was explored. The results demonstrate that all temperatures are lower under a 21% O-2/79% CO2 atmosphere than those under an air atmosphere. In oxy-coal combustion, the increasing oxygen concentration can increase CO emission. NO emission in oxy-coal is much lower than that in air combustion as a result of coal particle combustion generating less oxygen status under a 21% O-2/79% CO2 atmosphere. The percentage of mercury oxidation is higher under air-coal combustion than that under 21% O-2/79% CO2-coal combustion, because the specific heat of CO2 is higher in 21% O-2/79% CO2 than that in air. The increase of the oxygen concentration under oxy-coal combustion leads to the total reducing atmosphere decrease, which is suitable for the mercury oxidation. These results are very significant for selection of operating conditions for an oxy-coal circulating fluidized bed.