As one of the most promising approaches to reduce CO2 emissions from power plants, oxy-fuel fluidized bed combustion technology still has a lot of basic uncertainties requiring in-depth studies, especially in application for coal and biomass co-firing. This work focuses on results of coal co-firing with biomass under O-2/CO2 combustion conditions in a 6 kW(th) bubbling fluidized bed combustor. With the same thermal input maintained, computational fluid dynamics (CFD) was adopted as a predictive tool to examine the effects of firing coal and co-firing biomass with coal under O-2/CO2 atmospheres. Hydrodynamics, heat transfer, species concentration, and char gasification in a fluidized bed were investigated. In comparison to coal combustion, the hydrodynamic and temperature profiles of biomass co-firing are not affected much when maintaining the same thermal input. The temperature of biomass co-firing at the bottom of the dense-phase zone, which is mainly the char combustion area, is about 10 K lower than that of coal combustion. Higher O-2 and H2O and lower CO2 outlet concentrations were detected with biomass co-firing. Moreover, some typical parameters in biomass co-firing under oxy-combustion, including the cross-sectional average pressure, volumetric fraction distribution of the solid, velocity, and flue gas temperature, were analyzed in different atmospheres (21% O-2/79% CO2, 30% O-2/70% CO2, and 40% O-2/60% CO2). These results provide auxiliary insight into the complex influences on the biomass co-firing process by different atmospheres. Simultaneously, the experimental study of biomass co-firing under different atmospheres is used to validate the CFD modeling.