The parallel packed bed reactor (PPBR) is a promising route for industrial application of chemical looping combustion. This paper introduces a two-step design approach for it. The preliminary results quickly obtained from the phenomenological design are used as input parameters for iterative modifications using the numerical design, striking a good balance between cost and accuracy. The phenomenological method is established by modeling the PPBR based on the macroscopic propagation characteristics of the heat and reaction front over different time scales. Based on it, a syngas-fueled 1.5 MWth reactor is designed, and the implicit relevance among design parameters is explored. Results reveals that the height-to-diameter ratio of the bed must meet certain constraints to ensure stable and cost-effective operation, and these constraints are also extended to the scale-up process. Sensitivity analysis of design parameters and the effects of particle size on intra-particle gas diffusion are analyzed to determine the preliminary design schemes. Then, the numerical design method based on the one-dimensional homogeneous model is employed as an extension and supplement to the phenomenological design. Comprehensive identifications and analyses of the dynamic characteristics of the transport-reaction phenomena in each phase are conducted. By analyzing the difference between the simulation results with the preliminary design schemes, as well as the continuous operational stability of the unit, the reliability of the phenomenological design is validated. Finally, the preliminary design schemes are further optimized to enhance the energy output under the consideration of operational compatibility for different modes like different inlet modes and reactor modes.