Oxy-fuel combustion, O2/H2O combustion has many advantages over O2/CO2 combustion, and has gradually gained more and more attention recently for carbon capture from coal-fired power plant. The unique physicochemical properties (thermal capacity, diffusivity, reactivity) of H2O will definitely influence the coal combustion process. In this work, the combustion characteristics of lignite char were investigated in a fluidized bed combustor under O2/N2, O2/CO2 and O2/H2O atmospheres with different oxygen concentration (15 %-27 %) and bed temperature (837-937 o C). Results indicated that the average reaction rate (raverage) and the peak reaction rate (rpeak) of lignite char in O2/H2O atmosphere was lower than that in O2/CO2 atmospheres at low oxygen concentrations. However, as the oxygen concentration increased, the rpeak and raverage in O2/H2O atmosphere was significantly improved and exceeded that in O2/CO2 atmosphere. The calculation of the activation energy based on the shrinking core model showed that the order of activation energy under different atmospheres is: O2/CO2 (28.96 kJ/mol) > O2/H2O (26.11 kJ/mol) > O2/N2 (23.31 kJ/mol). Furthermore, with the increase of the bed temperature, the active sites occupied by gasification agent were significantly increased, the active sites occupied by oxygen decreased correspondingly.