For the dual circulating fluidized bed reactor of chemical looping combustion (CLC), challenges like reasonable oxygen carrier (OC) fluidization/circulation, efficient fuel conversion, and stable autothermal operation are critical bottlenecks that need to be addressed. Rational design of the reactor is important to overcoming these challenges. This paper employs a two-step joint design process combining engineering and numerical method, to design a coal-fueled 5 MWth iG-CLC reactor. Initially, a self-consistent engineering design method is established, clarifying relationships like mass/energy conservation, fuel/OC reaction, and fluidization characteristics within the bed. It is used to investigate the control of OC circulation, and the corresponding heat/mass transfer, delineating a reasonable range of these issues. Also, the impact of main design parameters on fuel conversion and operational costs are explored, and sensitivity analyses are conducted, leading to a preliminary design scheme for the 5 MWth reactor. Subsequently, preliminary design of air/fuel reactor are numerically validated and optimized by one-dimensional macroscopic model. Gas-solid fluidization are examined, focusing on the distribution of superficial gas velocity, bed materials, and bed pressure. In terms of reaction states, the distribution of gas species and reaction rates in the dense and dilute phases are analyzed, revealing the conversion patterns of fuel and OC in different regions. For the autothermal operation, this study analyzes the distribution of gas and solid temperature profiles across the bed, investigating the heat circulation and transfer mechanisms. Finally, based on these analyses, the preliminary design schemes are optimized to improve gas-solid fluidization and fuel conversion.