Spout deflection is a common instability phenomenon in spout fluidized beds. However, spout deflection was not well characterized and quantified in the past. In this work, spout deflection in a flat-bottomed spout fluidized bed is investigated at the particle scale by means of Computer Fluid Dynamic-Discrete Element Method (CFD-DEM) coupling approach. Two types of spout deflections - alternating and non-alternating spout deflections are captured by the CFD-DEM simulations and qualitatively analyzed in terms of voidage distributions and particle momentum profiles. The spout deflection angle is defined and used to characterize the intensity of spout deflection quantitatively. Then, the effects of spouting velocity on alternating spout deflection are investigated quantitatively using the spout deflection angle. The simulation results indicate that spouting velocity has a limited effect on the amplitude and frequency of alternating spout deflection. The relation between solid flow pattern and spout behaviour are also investigated with the help of spout deflection angle. The results indicate that spout deflection angle is highly relevant to particle distribution non-uniformity at the upper region of the bed. The study offers a cost-effective tool to characterize and quantify spout deflection in a spout fluidized bed. (C) 2019 Elsevier Ltd. All rights reserved.