In order to study the hydrodynamics characteristics of gas-solid two-phase flow in the two-stage series riser, a filtered two-fluid model considering filter size, filtered solids volume fraction and slip velocity is used. Comparing to simulated results from different drag models including the filtered model, Gidaspow model and Wen-Yu model, the results from the filtered model are in better agreement with the experimental data of Zhu et al. Also, the details comparison of flow behaviors of particles between the two-stage series riser and typical riser are conducted. The enlarged section in the bottom for a two-stage series riser can strengthen the back-mixing of particles and gas-solid interaction. Simulation results reveal that the solids volume fraction displays the typical S-type distribution with an increase of solids mass rate, but the gap is declined along the bed height. The distributions of solids volume fraction, axial velocity and residence time are investigated by analyzing the simulated results at the different geometry of enlarged zone at the bottom of a two-stage series riser. (C) 2019 Elsevier B.V. All rights reserved.