Abstract: The internal characteristics of a hot fluidized bed reactor exhibit complex gas-solid reaction flow features, including the distribution of gas components, gas velocity, particle composition, particle velocity, pressure distribution, and temperature distribution within the bed. However, due to experimental conditions and measurement techniques, these parameters are often difficult or impossible to measure through conventional means during the actual operation of the reactor. To gain a deeper understanding of the key operating parameters and characteristics within the fluidized bed reactor, to master its operational control laws, and to identify optimization issues in reactor design and operation, it is essential to employ numerical simulation methods for research. Furthermore, numerical simulation can bypass the lengthy process of specific structural design and experimental system setup, thereby accelerating the iterative promotion of the device and significantly reducing the costs associated with design and construction. This study systematically models, simulates, and analyzes a 10 t/d sludge and biomass co-incineration fluidized bed reactor using the Computational Particle Fluid Dynamics (CPFD) method, focusing on the fluidization and reaction states of the reactor under both closed and open secondary air conditions. The simulation results indicate that the reactor operates normally under rated design parameters, with reasonable gas-solid fluidization and high fuel conversion rates, particularly with minimal fuel slip detected in the flue gas. In the study of the fluidization state, when the secondary air is closed, the bed material reaches stable fluidization within 2 seconds, primarily concentrated in the lower part of the reactor. The concentration of particles in the dilute phase decreases with height, with no significant particle outflow observed. During stable operation, the reactor exhibits a core-annulus structure, with the solid content in the center lower than that at the wall. The pressure is highest at the air distribution plate, and significant pressure fluctuations occur at various measurement points after fuel addition, indicating intense combustion disturbances. The gas-solid velocity distribution shows that the gas velocity is high and disturbed in the lower region, while it is slower and less disturbed in the upper region, leading to vigorous fluidization of the bed material. When the secondary air is opened, the total air volume remains constant, resulting in a decrease in primary air and an increase in secondary air. This leads to a reduction in the height of the dense phase region and an increase in particle concentration, exhibiting a bubbling fluidization state. The airflow velocity and disturbance level significantly decrease, while disturbances near the upper secondary air inlet increase, creating a counterflow state. Regarding the reaction state of the reactor, when the secondary air is closed, the fuel rapidly undergoes pyrolysis and combustion, with char burning out quickly and ash concentrated in the lower bed layer. Gaseous fuels (such as CH₄, C₂H₄, CO, H₂) primarily combust above the feed inlet, resulting in high concentrations of CO₂ and H₂O, with O₂ nearly depleted. Nitrogen pollutants (HCN, CNO, NO) mainly convert in the middle and lower sections, while sulfur pollutants (H₂S, SO₂) are distributed in the upper part. The high-temperature zone expands in the upper section, reaching a maximum temperature of 1217 K, and carry out pilot test verification, the maximum temperature in the furnace during the test is 1163.15K with an error of 4.6%, which is within the acceptable range.. The temperature in the bottom dense phase region is uniform, and the gas-solid convective heat transfer rate is high. After opening the secondary air, the oxygen supply from the primary air decreases, leading to an increase in gaseous fuel concentration and an expanded distribution area, limited to below the secondary air flow path. The disturbances and oxygen supply from the secondary air promote rapid fuel burnout, resulting in a more uniform distribution of pollutants CNO and NO, while high concentrations of SO₂ are concentrated at the top of the reactor. These research findings provide guidance for the analysis of fluidization and reaction states in sludge and biomass co-incineration fluidized bed reactors, as well as for the structural design and optimization of reactors.