In this study, the flow stability of the flat-bottomed hopper was investigated via GPU-based discrete element method (DEM) simulation. With the material height inside the hopper reducing, the fluctuation of the flow rate indicates an unstable discharge. The flow regions of the unstable discharge were compared with that of the stable discharge, a key transformation zone, where the voidage showed the largest difference between unstable and stable discharge, was revealed. To identify the relevance of the key transformation zone and the hopper flow stability, the voidage variation of the key transformation zone with material height reducing was studied. A sharp increase in the voidage in the key transformation zone was considered to be the standard for judging the unstable hopper flow, and the 'Top-Bottom effect' of the hopper was defined, which indicated the hopper flow was unstable when the hopper only had the top area and the bottom area, because the voidage of particles in the top area and the bottom area were both variables. (C) 2017 The Chemical Industry and Engineering Society of China, and Chemical Industry Press. All rights reserved.