The Computational Particle Fluid Dynamics (CPFD) method was applied in the simulation of the gas solid flow in a circulating fluidized bed (CFB) with a loop seal in this work, and the modeling parameters were mainly investigated in this part. The influences of some crucial modeling parameters in CPFD simulation, such as mesh size, particle close pack volume fraction, interphase drag model and particle size distribution (PSD) on the flow behaviors in the whole loop of CFB were carefully analyzed with the help of experimental data conducted on the same CFB test rig. A medium mesh size was proved to be appropriate and used for the CFB modeling in this work. The particle close pack volume fraction largely affected the particle packed condition in the dense loop seal and standpipe. A value of 0.58 was proved to be suitable to describe the particle circulation and pressure distribution characteristics. Different gas-solid drag models showed certain influence on the particle circulation behaviors, and the combination of Wen-Yu and standard Ergun model was determined as the optimum choice. The consideration of the real PSD in CPFD scheme would better predict the overall flow characteristics for CFB system than the mono-size simplifications. The determination of modeling parameters in this work provides the base for simulating the gas-solid flow in the whole loop of CFB reactors by CPFD method, especially for the dense flow behavior in the standpipe and loop seal. (C) 2013 Elsevier B.V. All rights reserved.