Abstract: Biomass oxy-fuel combustion in fluidized bed can achieve negative carbon emissions over the entire lifecycle, making it one of the key technologies to ensure the achievement of China’s “dual carbon goals”. However, the prominent issue of uneven mixing of oxygen and fuel in biomass fluidized bed boilers, along with the high energy consumption of oxy-fuel combustion, has limited the further development of this technology. The oxygen carrier aided combustion technology can improve the spatial and temporal distribution uniformity of oxygen in the fluidized bed, thereby enhancing combustion stability and efficiency. The combination of oxygen carrier aided combustion technology with biomass oxy-fuel combustion in fluidized bed−oxygen carrier aided biomass oxy-fuel combustion—has the potential to improve oxy-fuel combustion efficiency and oxygen utilization while reducing the energy consumption of oxy-fuel combustion, and promotes the further development of this technology. However, research on the conversion mechanism of solid fuels at the conditions of oxygen carrier aided oxy-fuel combustion is still insufficient. Using a visualized fluidized bed experimental setup, combustion experiments of single biomass and bituminous fuel particle were conducted at different bed materials, atmospheres and various operating conditions. While observing the particle combustion behavior, the temperature of the flame and char particles was monitored using two-color pyrometry. The effects of active bed materials on the ignition process, volatile release and combustion process, and char combustion process of biomass and bituminous fuel particle were systematically evaluated. The results show that compared to inert bed materials, oxygen carriers have little effect on the ignition and devolatilization processes of biomass particles, due to the relatively large Biot number of biomass particles and the fact that the devolatilization process always occurs on the bed surface. But it reduces the ignition delay time and increases the devolatilization time for bituminous coal. The peak temperature of char particles increased by about 30 K when the bed material is active, while the burnout time exhibited different patterns depending on the type of fuel.