Vertical slot fishways are structures that facilitate energy dissipation using a specific pool, slot and baffle design; these structures are common in West China. To develop an effective and simple design, the flow patterns in the pools must be fish-friendly. To assist in design optimisation, a three-dimensional (3D) computational fluid dynamics (CFD) model and an experimental study for validating the model using an equivalent scale physical model were conducted. This paper presents the results for detailed hydraulic structures, including velocity fields, flow patterns, recirculating flows and turbulent structures in a pool, for conditions with slopes of 4.2% and 2.6% and a new structure with a long 'L'-shaped baffle. The new structure can effectively improve the flow patterns of fish by adopting a burst-coast mode. The mainstream velocity decreases from 1.22 m/s to 0.85 m/s, and the average proportion for the recirculation region decreases from 30.16% to 21.73% compared with the same slope and creates a 'Omega'-shaped flow pattern. The results provide insight of the flow patterns in vertical slot fishways, which can be used to guide future designs. (C) 2016 International Association for Hydro -environment Engineering and Research, Asia Pacific Division. Published by Elsevier B.V. All rights reserved.