Flexible filamentous particles have been widely applied in the industrial fluidized drying process. The hydrodynamic characteristics and equipment drying performance will be greatly influenced by the inhomogeneity of the special particle flow. So it is of great importance to gain more insight into this cluster phenomenon. In this paper, a new measurement algorithm is used to study the cluster distribution characteristics of the flexible filamentous particles in a cold fluidized riser. Specifically, influence of the particle length, moisture content and operating parameters on cluster distribution is emphasized. Moreover, the distribution of solid volume fraction inside the clusters is studied. The experimental results are compared with the clustering characteristics of spherical particles. Last, the correlation of the average cluster diameter and the frequency are formed. The results showed that the particle length, moisture content and operating condition has a significance influence on the fluidization behavior and the distribution characteristics of clusters. This work would provide an important reference for the drying process of this particular particle. All these will benefit the design and scale-up of related equipment. (C) 2018 The Authors. Published by Elsevier B.V. on behalf of Institution of Chemical Engineers. This is an open access article under the CC BY license.