Cylindrical particles have been widely used in fixed bed reactors for their better mechanical strength by extrusion granulation. The size of cylindrical particle often varies each other in industrial application. How size distributions and packing rates of particles influence the packing structures and flow behaviors in fixed bed are still not clear. In present study, Discrete Element Method (DEM) was employed to simulate packing processes of cylindrical particles. The influences of particle size distributions and packing rates of particles on packing structures of fixed bed were investigated. Subsequently, local fluid flow behaviors and overall pressure drops inside fixed bed were studied in detail by Computational Fluid Dynamic (CFD). It can be found that the wider the size distributions are, the bigger the bed porosities are. Smaller packing rate of particles is beneficial to obtain denser packing beds. The radial porosity distribution is not uniform only near the wall region for wide size distributions. The local non-uniform flow exists and the average axial velocity shows similar distribution tendency with the local porosity. Pressure drops along the bed were obtained based on detailed flow simulations and compared with empirical correlations. The equation parameters were modified for smaller deviations based on the CFD results. The present study gives a detailed understanding of packing process for polydisperse cylindrical particles and the fluid behaviors inside fixed bed, which is very useful for guiding industrial reactor installation and reactor design. (C) 2019 Elsevier B.V. All rights reserved.